A lens that nobody has mentioned, that I personally recommend, is the EF 17-40mm F4L.
The reason I choose this lens is that it is close to the 24-70L range on a Full Frame camera.
It'd be roughly 27.2mm - 64mm after taking into account the 1.6x sensor crop. The reason this is a particularly good lens is that there is no vignetting at all at the edges even when wide open on a 1.6x crop sensor like the Rebel XT or the 20D.
Another good reason to purchase this lens is that it goes for roughly ~575 on the second hand market, and also, you will rarely, if ever, shoot at anything larger than F4 as it is in a fashion shoot as depth of field would be too thin to showcase the clothing you want to put detail into.
However, if you want to shoot birds, or or something, then go ahead, knock yourself out and get like a 300mm F2.8L IS or something similar.
If you want to take portraits with decent bokeh, maybe a 50 1.4, 85 1.2/1.8, or 135 F2L.
Otherwise for a budget conscious photographer, the 17-40L is quite decent.