I have the older Sigma 10-20mm f/4-5.6 EX DC HSM lens. This lens may not take in as much light as other lenses, but when I bought it I found a really good deal on it. Again, this is not the newer (more expensive) f/3.5 version.
If you need more light, you could always add an external flash (that you can then use with other lenses).
While I would have preferred the Canon or Tokina, what made me decide for this Sigma was when I found two coupons I was able to stack.
This was a case of it having good enough image quality (from various reviews it is close to the other two) and being a lot cheaper at the time.
The difference between 11mm and 10mm doesn't sound like much on paper, but it is in practice. It doesn't go in as far as the Canon, but I have other lenses for that.
A really nice thing about this lens over the Tamron, or even the Canon kit lens, is HSM. Basically it is Sigma's answer to USM. It is a faster focus without front end rotation. It is also silent. (I don't recall if the tamron front end rotates or not.)
While it is unbelievably wide for me, it may not be for someone else.
To give you a perspective, this sigma gives you all that you can see at (roughly) a 102.4º at the wide end. What this means is to get a math compass, set it to that angle, and try look through it. Anything between the two ends is what it will capture. If that is not wide enough, then you need to get something wider.
Canon Rebel XTi, BG-E3 Grip
Canon EF 16-35mm f/2.8L II, 24-105mm f/4L, 50mm f/1.4, 70-300mm f/4-5.6 DO, 85mm f/1.8
Sigma 150mm F2.8 EX Macro
Sigma 10-20mm f/4-5.6 EX, Canon EF-S 18-55mm f/3.5-5.6 II
Canon 430EX II
Opteka 13mm, 21mm, and 31mm extension tubes