Canon EF-S 10-22mm f/3.5-4.5 USM
This lens fills the essential ultra wide angle focal range for APS-C cameras (not compatible with full frame cameras.) Effectively 16-35mm due to the crop factor, this lens can help you capture those expansive landscape scenes with great breadth and depth to them. The aperture is non-fixed and rather narrow for a maximum aperture, however with landscape photography, this will usually not be a problem.
Not recommended unless you only use APS-C cameras, and never intend to use a full frame camera. Useful focal range, but limited range of use due to aperture.