I agree with brent. The 50mm f1/8 can NOT be beat for the money. Very fast, uber sharp, and very cheap, every photog needs one, no matter how big or small they are. I also agree that a 16-35L or the 17-40L are great wide angle lenses that are not too terribly expensive. Also, if you can afford it, get a nice telephoto lens as well. I suggest 100-400L if you are primarily shooting outdoors, or the 70-200L is a great lens in any version, especially if you can afford the 2.8 IS version.
If money is an issue, (students, most likely) as noted above, the 50mm 1.8 can not be beat, hands down, must get! Also I would recommend if you have not bought your camera yet, Cosco has a XTI kit that comes with a 18-55 IS, and a 55-250 IS for around 1000 bucks.
So:
50mm 1.8 - Must
16-35L or 17-40L Very good, more $$$