Sigma 10-20mm f/4-5.6 EX DC HSM is a real bargain for an ultrawide zoom for sub-frame cameras. If you want to shoot really wide with your DSLR, this lens looks like a good way to go about it.
What's it good for?
While the "traditional" uses of wide-angle lenses are in landscape and architectural photography , wide-angles are by no means limited to this. In particular, WA's are excellent for interior and situational/street photography, and have their uses even in portraiture.