In reply to NathanW:
I have a D5000 and went down the 10-20mm route, and got the Sigma f3.5. For me, it was more about getting right into the action and still retaining context, rather than wide vistas which it sounds like is your objective. Having said that, I've taken a few landscapes with it too.
If I were to buy wide again I'd definitely go for the Tokina 11-16mm f2.8, the Sigma's too blurry at the egdes, especially when (close to) wide open. In fact, the Tokina was recommended to me on here by several, but I just wouldn't listen. :-/ It saved me only a few quid by going with Sigma. Big mistake, although the Sigma's inbuilt a/f does work with my body.