Well no doubt that the Sigma 10-20 is able to produce excelelnt results, there are however quite a few other options as well.
Primes:
DA14:
Excellent performer, excellent lens in all respects (one of my personal favourites)
and resonably fast (f2.8)
Go here for a number of samples:
Zenfolio | Duplo Photography | Pentax DA 14mm f2.8 IF Zenitar 16mm Fisheye:
Seems to be performing quite well too and has a lot of followers here.
Full manual lens though.
Tamron 14mm:
Sean (Carpents) has one and really likes it.
Zoom (aside from the 10-20)
Sigma 12-24:
I have heard good about it, but is a big and heavy bugger of a FF lens.
DA12-24:
Seems to be producing excellents results, I have no deep personal experience with it though.
Constant f4. a tad slow at night though.
DA10-17FE:
A very fun well constructed and overall excellent lens. similar FOW to the DA14mm at 17mjm.
A load of examples can be found here:
Zenfolio | Duplo Photography | Pentax DA 10-17mm f3.5-4.5 Fisheye
Probably a lot more around, but those were the "off the top of my head" ones...
My personal favourites are the DA14 and DA10-17 in the range, but that is a very personal preference.