Joseph Wisniewski , May 12, 2011; 05:36 p.m.
Velbon mag slider. Honestly, it's cheap, reliable, and accurate. Remove the lateral (short side-to-side) rail, you won't use it much, if at all, and the main rail is much more stable without the extension. About $100.
I also am rather partial to the Novoflex Castle-L, but it does cost over $300.
In general, I avoid the Chinese generic rails (eBay, deal Extreme, Adorama, Kirk) as too tall, not having decent bearing blocks, and overall, wobbly. Rails are like sports cars, they're stable when they're low and wide, not tall and thin.