The suggestion of a refurbished Canon EF 17-40mm f/4L USM is a good one, you could also look for a good condition used one as well (not necessarily refurbished). Adorama sells used equipment:
http://www.adorama.com/catalog.tpl?o...gory&cat1=Used
Unless you plan on upgrading to a full frame sensor, I would also look into a used or refurbished Canon EF-S 17-55mm f/2.8 IS USM - it's naturally not as sharp as an L lens but it's supposed to be a very good quality lens and you'll get the extra fstop and IS, if you want them. Just depends what is a bigger priority to you. It's about the same cost as the L lens though (slightly above your budget new), which is why I recommend used or refurbished.
On the third party (and thus less expensive so you could buy new) front, there's also the Sigma 17-50mm F2.8 EX DC OS HSM but I agree that the Tokina suggested by Ray is a little bit sharper (though it's actually 16-28mm, not 24). The Sigma is slightly cheaper but only by about $50 I think.
If you're looking to go even wider, your main options for a fast lens would probably be the Tokina 11-16mm f/2.8 AT-X Pro DX and the Sigma 10-20mm f/3.5 EX DC HSM - they seem about the same in terms of sharpness but the Sigma appears to have more vignetting. But neither as are sharp as they 16mm+ lenses suggested.