Tamron 17-50mm f/2.8 XR
For around $400, it's easily the most versatile, best "bang for the buck" lens for APS-C cameras .
I kinda want to sell my 24 - 105 f/4 L IS and buy 2x primes
Canon Super Wide Angle EF 20mm f/2.8 USM
or
Canon Wide Angle EF 24mm f/2.8
+
Canon Normal EF 50mm f/1.4 USM
From all the reviews I've seen, the 20mm f/2.8 is a less than stellar lens. That, and it's neither very wide nor very fast (it's ~32mm on a crop body). 17-55 IS or the Tamron 17-50 seem like they would be better options since both of those lenses will give you 17, 20, 24, 28, 35 and 50mm, all with f/2.8 aperture (and IS in the case of the Canon).