Canon EF-S 10-22mm f/3.5-4.5 USM
Most photographers use ultra-wide angle lens for sweeping landscapes. I love to use mine for architecture, cityscape and even candid.
Here is my review of this amazing ultra-wide zoom:

Center Sharpness	Excellent, resolution and sharpness, similar to my 17-40L, even wide open at f3.5 Its sweet spot is somewhere between f5.6 and f8
Edge Sharpness	A bit soft and lower contrast when wide open. Not as good as my 17-40L on a 1.6 crop body. Greatly improve when stopped down to f5.6
Distortion	Surprisingly good for an ultra-wide angle lens. Even better than my 17-40L. For critical architectural shots, it is easily correctable by PT Lens. However, low rectilinear distortion does not mean the absence of perspective distortion associated with all wide angle lens. See my write up here.
Chromatic Aberration (CA)	CA is the weakest link of this otherwise excellent lens. CA is noticeable, more than 1 pixel wide in the magenta/green, especially on the wide end of the zoom. But all ultra-wide lens have this problem. Adobe LightRoom can do a great job of removing CA. At 10mm, a red/green setting of around -35 works best. I don't see any yellow/blue CA.
Flare Resistance	Excellent After 2 years of heavy use, I did not have a single shot ruined by flare. It is better than my 17-40L and a lot better than my 17-55 IS. I use the hood that came with the 17-40L, not to prevent flare, but to protect it from elbows in crowds.
Auto-Focus	AF by USM is fast and silence. With its DOF, focus is always accurate.
Build Quality	It is not an L lens. Smaller and lighter than the 17-40L and all its 3rd party alternatives.
Price	It costs more than lens from Sigma or Tokina, but you get consistent QC. Before you complain, check the price of Nikon’s 12-24mm.