Steve Fuller of Kelowna, BC recently pointed out to me that after many years of using the Sigma 10-20 f/4-5.6 that he was moving to the Tokina 11-16mm f/2.8. The Tokina 11-16mm lens is specifically designed for APS-C sensors (cropped sensor DSLRs). It's been on the market since about 2008.
As Ken says in his review the Tokina 11-16mm is probably the best wide angle lens for Nikon DSLRs but for Canon DSLRs the Canon 10-22mm is probably a better bet.
I have the Tokina 11-16mm f/2.8 listed in my real estate photography lens table. It is shown with a lower rating than the Sigma 10-20mm simply because the slrgear.com site that I use for lens ratings rates it lower. It could very well be that the Tokina 11-16mm is a better lens than the Sigma 10-20mm. It is a little more expensive.
Is there anyone else out there that have used both the Sigma 10-20mm and the Tokina 11-16mm? Which do you like best?