Nick Bumgardner , Oct 20, 2007; 06:55 p.m.
Well I think the most common filter is a UV filter. A UV is often times sold as an upsale on camera lenses. People debate the relative importance of UV filters and you can read about that on this site.
Other common filters are polerization, red, yellow, 81a, 81b, cross cut,ND Grad, not to mention many others. Polerization filters work to reduce glare but also take away at least a stop depending on the one used, red and yellow are generally only used on black and white film. 81a and 81b are often times used on slide film to give it more pop.
My suggestion is that you shoot and shoot and when you come to a problem get a filter if required to fix it. You will learn what you need by shooting more and more.