IMHO, for your applications, 70-200 f/4L and 100 f/2.8 Macro. The former for sports, the latter for portraits, general use, and of course, macro.

Congrats to your new toy! :)
I would recommend the EF 100/2.8 Macro, razor sharp, relatively cheap (great "bang for the buck"), and works like a charm for both life-size macro work and portraits (at least outside on a crop camera like the XSi). I think the AF is quick enough too for shooting sports even if longer lenses would sometimes be preferrable of course, same for the zoo. But as a compromise if you could only have one lens for the applications you describe, that's my 2 cents...and it's almost twice as long as your 18-55, which should be good enough for the museum, with IS and all.
Good luck!

Aaha you meant 2 lenses other than your kit lens...then I second Steves suggestion with the 70-200 as well..

Low budget:
One lens: Just the 55-250/IS, which is cheap, a long zoom, has a magnification of 0.31x which is pretty big to begin with and which can be enhanced with a Kenko set of tubes or a 250D/500D for macro.

Higher budget:
The same 55-250/IS plus either the EF-s 60/2.8 macro of the 100/2.8.

High budget:
One of the 70-200 L's (The 4's are great and the 2.8's are the fastest but are huge beasts.) or even a 100-400L. These lenses are also great for portraits. The 100-400 is pretty big though and the gap between your 18-55 and a 100 is also pretty big.
Plus the 100/2.8 for macro.

Even higher budget:
Three lenses... (One of the sets above plus a fast prime like a 50/1.4, 85/1.8, 100/2 or 135/2 for portraits.)

Kind regards, Matthijs.

Nikon 14-24 f/2.8
and
Leica Noctilux 50mm f/0.95

:)
EDIT: Sorry did not read the topic to the end :)

zoom: 70-200 2.8 IS
macro: 100 macro

On a budget, I would get the 50mm f/1.8 for portraits.

Real world money:
I have the sigma 70mm f2.8 DG macro and am delighted with it. Nice portrait lens on an APS-C body. Will also work of FF cameras.
70-300mm IS (non DO) a better alternative to the 55-250 IMHO in that it can work on full frame bodies as well. A bit more future proof. Fast AF, metal mount.

Bit more money:
Sigma 70mm DG (can't see past it)
Canon 200mm f2.8 L II A fast focusing sharp ripper of a lens. Not as flexible as a zoom, but this in my experience makes it faster to use. You have one less parameter to think about. Not all that expensive for the quality.

Keeping your budget in mind: For zoo, soccer and portraits with OOF backgrounds in strong light, 70-300 IS (non-DO). For available light and OOF backgrounds in bust-length or longer portraits, the 50mm/1.8 (which turns into an 80mm (35mm equivalent) on your camera These lenses will interface nicely with your 18-55 for your stated uses. Next, I would add a Sigma 30/1.4. For macro, add a good Canon diopter lens to that 50mm.

For the telephoto I agree with the suggestion of the Canon EF-S 55-250 4-5.6 IS. This is an unusually sharp lens for the price range which is in the 'quite reasonable' bracket. For Macro it depends how macro you want to go. Macro officially means a magnification of at least 1:1 or the same size on the sensor as in life. In this bracket the Canon 100 f2.8 macro is very good but the Tamron 90mm f2.8 and the Sigma 105mm 2.8 are close behind.

As an alternartive there are a number of lenses which call themselves 'macro' which should more properly be called 'close focus'. For example the Sigma 24-70 f2.8 Macro doesn't give 1:1 magnification but works quite well for gwetting closeups. Check manufacturers figues on magnification and min focus distance here.

Another option is to also get an extension tube which enables you to get closer to a subject with a normal lens. These come in different lenghts such as 12mm and 25mm. They are just empty tubes with no glass in but which have all the bayonet contacts so the camera and lens work as normal. These are preferable to close up diopter lenses as they don't add to the amount of glass being used.

My suggestion is that you add lenses in two steps. A telephoto zoom will take care of most of the needs you have. Whether you choose a 55-250 or 70-300 (both with IS) is up to your budget. Both will be fine for close-ups of all kinds of things. Maybe that's all you need, or maybe you'll find that you'd be better served by a 50/1.8 ($80) for extravagant indoor portraiture and added low light capacity. Only add a true macro lens if you do find that your lens's close-up capabilities limit you. Any of those suggested are excellent. Most current macro lenses are.

I'd vote for the 100mm 2.8 macro. You get a sports telephoto, a macro and a decent portrait lens all in one.

Obviously there is no one combo that everyone agrees with....it all depends on your budget. My 2 cents worth, get the 200mm 2.8L lens, awesome clarity for under $700, really will be needed for soccer and descent shots of the animals at the zoo. Also works very well in almost any low-light situation. If your budget is tight, the 200 L will also double as a descent macro lens.
The second lens I'd suggest is the sigma 30mm 1.4, which will give you the great portraits, and work well in even lower light. It is just under $400. If that is too steep, go with the 50mm canon for around $90.
I'd agree that the 100mm 2.8 would be a good choice. Though the 100 alone isn't long enough for all kinds of sports. The 100mm macro will set you back around $450, and I would count it as a third option after the other two.

Canon 55-250/4-5.6 IS is a good general purpose telephoto lens. It'll work really well for the zoo, ok for soccer (expect to miss some shots due to missed focus), and ok for flowers and bugs. Also an ok lens for easy wildlife and backyard birds.

Canon 85/1.8 is a fast portrait lens. It's one of the best lenses for selective focus and it's razor sharp (your "crystal clear with the background blurred"). It'll be ok for sports (blazing fast focus but it doesn't zoom) and ok for bugs and flowers.

Tamron 90/2.8 is a true macro lens. Great for portraits with selective focus, excellent for bugs and flowers, not great for sports (no zoom, very slow autofocus).

I have these three lenses and I recommend each of them. The 55-250 is a jack-of-all-trades-master-of-none lens while the primes are more specialized. Note that there is no one afforable lens that does it all - perhaps the closest is Canon 70-200/2.8 IS but this lens costs a lot of money.

Thanks again for great advice.
Is this the 70-300 IS y'all are speaking of?

Canon EF 70-300mm f/4-5.6 IS USM Ultrasonic Telephoto Zoom Lens

Canon 100-400mm lens and 50mm 1.8

Just keep in mind that if you ever need to shoot indoor sports, or anything at night, F4 is going to be too slow in almost every situation.

btw, sigma makes a 70-200mm f2.8 that is much cheaper than the canon, which is supposed to give pretty good results, not as sharp as the canon though. You could get it and the 50mm f1.8 for under $1000.

Congrats! You're off to a great start! :-)

Summer, the 70-300 IS (the cheaper, ordinary one, not the green-ring DO version) would be an excellent choice for sports and wildlife photography on a budget. You can get it about $100 cheaper from B&H, which is a very reliable source: http://www.bhphotovideo.com/c/product/397663-USA/Canon_0345B002_70_300mm_f_4_5_6_EF_IS.html
It's an extraordinarily sharp lens (giving the professional 70-200L lenses a run for their money) and will allow you to zoom in very well on the action. You'll have an uncovered gap between 55 and 70mm, but I suspect you can live with that for now. I think the extra 100mm will be very useful to you for soccer photography. That lens on a crop body (like the XSi) should allow you to frame up shots on the opposite side of the soccer field (whose width, interestingly, is proportional to the height of the players). I think 200mm would be a bit inadequate on the tele end. (The 70-200 lenses are otherwise beautiful lenses, to be sure!)

For fuzzy-background portraiture, you might want to play with the 50mm f/1.8 II lens, which will cost you under $100. However, you might be able to achieve what you want with the 70-300. Just back off a bit, and zoom in. Keep the aperture large, and I think you'll get what you're after.

if you could only have two lenses, what would they be?

I'll try to answer the exact question you asked, but if I could have only two lenses one would not be a macro. I would want the EF 17-40mm f/4 L and the 24-105mm f/4 L IS.
Of course, that answer tells you absolutely nothing useful in your process of deciding what lenses would be useful for your photography and with your camera, unfortunately...

If I could only have 2 lenses with an XSi camera I would have the two I have; 17-55mm 2.8 IS and 70-200mm 4.0 IS. If I couldn't afford those two lenses I would get the 17-50mm Tamron 2.8 ($400) and the Tokina 50-135mm 2.8 ($650). I could shoot almost any kind of events with those two lenses and may get them as back up to my Canon lenses. They have excellent image quality that you can do professional level work with. Good luck!

17-55IS and 100-400L, works for me!

For general purpose and nature I have the 24-105 and 100-400 lens. For low budget general purpose I second the 55-250 with a 100mm macro. When I started the 17-85 and 70-300 IS lens was also a good option. Congrats.

Tamron - the 18-250 f/3.5-6.3 Di-II. Yours for only $470! Very versitle, good quality lens. Great for soccer photos, also gives a nice wide angle option. Use that until you feel like you need something faster, and then get the Canon 50mm f/1.4 to supplement.

The Canon 24-70 alone might satisfy a lot of your criteria. It is I think the closest focussing of the "near-macro" canon lens, and it's f2.8 will help with out-of-focus backgrounds. On a crop body it will behave as (very) slightly wide to modest telephoto.

please canon fan ! 70-200 mm IS L 2.8 and 24-105mm IS 4L are good choice to cover you expect . marco just for fun later .

Must have 17-55 2.8 IS next would be 70-200 2.8 IS. 17-55 2.8 on the speedy 40D and the 70-200 2.8 IS on the better 5D. v/r Raz

Well, the simple solution for sports would be a 70-200mm zoom. I agree that a Sigma 2.8 70-200 would be great. I had one and it is as good as the Canon, I just never used it. Make sure you get one that has HSM, which is the same as Canon's USM. USM means Ultra Sonic Focusing, and this is what you need, if at all possible, in any lens. It is the best auto focus system, it is silent, sure and very quick. Also, you want a 2.8 aperture, as this will allow you to use high shutter speeds to freeze the action. For portraits, a regular 50mm 1.8 Canon lens will be fine. It is very cheap, about $60 or so. The perfect lens for portraits is the 100mm 2.0 EOS USM. It would be great, if you can afford it. I would also get a Canon EOS Elan 7 film body and use that with the 100mm for portraits and sports. The dirty truth of digital photography is that film prints much better, and for pictures of your kids, it is best to have negatives, slides, black and white and color real prints. You will still have them in 30 years, when all of your digital files have long since been lost deep inside a hard drive somewhere. Also, the quality is far superior. With EOS the AF system is so good many of your film shots will be "keepers."

It would have to be the one's i use most,CANON 100 F2.8 macro and the Canon 300 f4 IS prime lens ,the 300 would be my walk about lens but in the nature reserve !i love macro work and this canon 100mm f2,8 has to be a L class lens for macro work ,Canon must have for got to put L on it,there is no dought it is a superb lens, if i had to have just one lens it would have to be the canon macro lens,a lot more fun in using this lens !.

I refuse to use zooms. They are too heavy, slow, and to my eye, not as sharp. I have a 50 f1.4 for my Nikon, and the use of my wifes 105 Macro. The only other lens I might need would be an 18mm, which they do not make, as far as I know. To the guy who wants a Noctilux: sure, get the most expensive lens made. Only there are problems besides the price. The lens covers about half the viewfinder, it is heavy, and it is not sharp wide open. (These comments are about the f1. I have not tried the new version). Better off with a 1.4.

Should we assume a budget of around $600 total?

(In update, OP said if 70-300 is purchased, there won't be any other purchase for awhile.)

There's also no reason for the OP to replace the 18-55; with that budget, it should be complemented, not replaced. Keep it!

Instead of the 70-300IS for $550 (price at B&H), I think a much more suitable setup for OP - not for the veterans here who are recommending a lot of really expensive lenses! - would be the

50-250 for sports ($250)
and
85/1.8 for low-light shooting and for portraits with out-of-focus backgrounds ($350)

If you want to do "flowers and bugs" occasionally, don't buy an expensive macro lens; just get a 12mm extension tube for 80 bucks (less if you buy a non-Canon tube), clip it to the 85mm lens, and you'll be all set.

Some of the answers above reflect how responders to these kinds of questions all too often reply with "What I would get," not "What OP should get." Clearly a non-L zoom and good fast prime are what OP needs, not a $1500 zoom or an expensive prime!

P.S. I probably should have suggested either the $100 50mm/1.8 (cheap solution) or the $350 50mm/1.4 (sturdier solution) as alternatives to the 85/1.8; only OP can decide which will be more useful, 50mm or 85mm. If finances allow, the 85/1.8 and the 50/1.8 would bring the total (with the 50-250, I mean) to around $700 and would make for a very nice kit.

CZ 16-80 and SAL 70-300G , with these two I got it all covered with the best glass.

70-200 f/2.8 IS L

24-70 f/2.8 L

2 EOS lenses?

To the OP - I would recommend a 200mm f/2.8 L (~$650 USD) and a 28-75 f/2.8 Tamron zoom (~$350-400) - both very respectable lenses. The 200m will have you set for telephoto photography. The fast aperture will also come in handy in low light and will produce the beautiful backround blur (usually reffered to as "bokeh"). The Tamron will make an excellent "standard" lens because it covers standard to short telephoto focal lengths on your XSi and is also a "fast lens" so it should throw things out of focus like you want. The 18-55mm can be utilised for now as a wide lens if need be.
I personally would get a 35mm f/1.4 and 85mm f/1.2 if money were no object though. I shoot 35mm/full frame digital, so I'd have a slight wide and short tele. for 90% of my shots I don't need anything else.

I think that when OP said, "If you could only have two lenses" (a common photo-forum hypothetical that gearheads love to toss around) the intended question was "If you had my needs and a budget of $600-700, which two lenses would you get?"

Assuming the latter is true, many of the suggestions here are not of much help - e.g. Nikon lenses, Leica lenses, L lenses that cost $3000 a pair (two above this post).

As I said above, with a budget of $600-700 I'd go with the 50-250 for kids' sports, a 50/1.4, a 50/1.8, or an 85/1.8 for portraits and low light, and a 12mm extension tube for macro.

You have an 18-55 already. I would recommend a 70-200 (IS if you can swing it) to go with it for the Rebel. Then get some strobes.

Thanks a million and I apologize for not wording my question better. I love to read all the posts filled with great ideas. I learn so much here. I will let you know what I end up getting.

I have a Canon 40D and own these 2 lenses... EF 17-40mm f/4 L and the 24-105mm f/4 L IS. Honestly I could be content to only shoot these, of course like most I lust after other glass!
My opinion is that you might try to rent a lens that you think you are interested in or see if a friend has one you can shoot. Good luck

35 f-1.4 and a 135 f-2 and a 1.4x extender to make your 135 into a 189 f-2.8

Well, the zoom covers wideangle, then a 50mm 2.8 macro would cover portraits, bugs and flowers, and the 70-200 2.8 for sports.

Since you have the Rebel XSi - I would highly recommend the EFS 60mm 2.8 Macro (unbelievably sharp and great bokeh) and the Tamron 70-200 2.8 or (if you can afford it) Canon 70-200 2.8 for the sports captures. You might also try the 85 1.8 for sports. I take pictures of my kid's basketball game all the time with the 85 1.8 and they come out sharp. The lens is very fast too, for low light venues.

I could never part with my 70-200 4 IS; like Pamela I love my 17-40. I love primes too, and have a few, but if I had to live with two lenses on a desert island, it'd probably be the zooms.

For a two lens kit, the 70-300 f/4-5.6 IS USM lens is a nice addition to the XSi kit lens. It can focus close enough for shots of flowers and insects. The 100mm macro is great for true macro and can be used for portraits. For a more affordable alternative, there is the plastic fantastic, a cheap, noisy lens that is almost too sharp for portraits. It is made by Cosina and sold under several other labels, including Vivitar and ProMaster. It is the 100mm f3.5 macro lens that comes with a 1:1 adapter. Typical used price is under $75, new they run $100-120.

Wayne

I agree that the 55-250 IS would be a great low coast addition to your kit lens. I got one to try out and found it to be great.

I own most of the expensive lenses that have been recommended. They are generally large and heavy, made to survive an atomic blast. For traveling, I got a 55-250mm. It is not as heavy or as well made as some of the others, but it is sharp and it is serviceable. It produces very good color and contrast and it has IS which compensates for my unsteady hand when I'm excited. It is obviously intended to compliment the 18-55mm kit lens. No one has mentioned the 50mm f2.5 macro. This is not a true macro because it only focuses to 1:2. A 1:1 true macro fills the frame with something the size of a postage stamp, while this only covers two postage stamps. It works well for portraits, is very sharp and costs about $240. If you need a true macro at a latter date there is a special adapter lens made to work with it that converts it to 1:1. It also works with an extension tube, if you don't want to pay the price.