I'd second dang on that - I used to own a Sigma 10-20 and it was a great lens (OK it was on Canon but still a great lens for the money!). It can stretch and soften details at the edges slightly at 10mm but the field of view is fantastic! I only changed it because I went full frame. I've got a few samples on my galleries too - they're mainly on the Lake District one, I think...here's one:
http://www.pbase.com/dharden/image/69559135.jpgThe Sigma 12-24, if you get a good copy (which also applies to the 10-20 - always test a Sigma and be prepared to send it straight back if all is not well, for a replacement) is very sharp and quite highly rated for interior work on full frame bodies so should be even better on a 'crop' camera. However if you want to do landscape it's not particularly practical cos you cant front mount filters such as grads on it....
There's also a Tamron 11-18 which isn't a bad lens either, although the last tamron lens i had produced rather "yellow" pictures so i haven't bought one of theirs since....
Fish eye lenses are quite specialist. If you're after "normal" but wide views then you need a rectilinear lens such as the ones already discussed here...
HTH!