Here are a few options:
Tokina 11-16 f/2.8 (constant aperture)
Sigma 12-24 f/4.5-5.6
Sigma 10-20 f/4-5.6
Canon EF-S 10-22 f/3.5-4.5
My opinion? Depends on just how wide you want, but if you want a dedicated wide angle, go with the Tokina. I have used the Sigma 12-24, and while very nice, wide, and greater zoom range, the higher f-stop isn't as great for depth of field or low light stuff, and it's not constant aperture meaning as you zoom in, the aperture will change from f/4.5 to f5.6 (if you have it set at f/4). Of course, you can leave it at 5.6 and that won't happen, but you are sacrificing a one or two stops there.
Keep in mind that these are EF-S lenses, designed specifically for the APS-C chip in your camera. If you upgrade to a full frame camera you will have some major vignetting. If you're not going to upgrade any time soon, you should have great results with these.
Also, check out this blog:
http://philipbloom.net/2010/08/20/which-lenses-to-buy/
Hope this helps!