I read a good one on here a few years ago that I've always remembered when these types of posts come up though I can't remember who posted it. "What shot can't you get with your current equipment that you really need?" The answer to that question will determine exactly where to go.
For a general purpose wide, that can be used for anything in conjuction with the 3x on the T3i, a Tokina 11-16 f2.8 is about as good as it gets. On the T3i it gives very sharp images from 11mm to 48mm with the 3x.
knowing what you already have would be helpful... in any case, for ultra-wide-angle, that tokina 11-16 f/2.8 is the usual recommendation - if you want something that's not so wide, the tamron 17-50 f/2.8 non-VC - more here: http://www.similaar.com/foto/equipment/us_lensc.html
I still haven't needed an ultra-wide lens on my T2i; haven't wanted anything that looked wide-distorted/fisheye-ish yet.
I use an 18-35 nikkor AF (one of my go-to still zooms). It's 3.5-4.5 (3.5 on the wide end) and very sharp on the DX sensor. About $350 used if you shop around.
The Samyang 8mm is considered really nice, especially for DX sensors. You can get them for under $300.
The main reason to use a Tokina 11-16mm is for a lot of shots. I recently started shooting models and it was basically impossible to get them in full body frame with any other lens. IMHO the Tokina 11-16mm is a priceless lens.
You won't be sorry. We just used it on our last shoot after having it in hand for about a day. Great lens! I'd share the link if interested but don't want to just pimp the link if you know what you bought.
I believe Canon Rumors reported that Tokina is about to release a new version of the 11-16mm. Hopefully, the new version will address the flare and CA issues of the current version.
You might also consider the Sigma 30mm f/1.4 if you think that is wide enough for your needs. It's frustrating that Canon doesn't have any modestly priced fast wide angle lenses in the same vein as the Sigma 30mm.