Best wide angle lens for Canon T2i (550D), Canon T1i (500D), XSi (450D), 50D and 40D
In early post, Canon EF-S 17-55mm f/2.8 IS USM was selected as the best standard lens for Canon APS-C cameras such as Canon T2i (550D), T1i (500D), XSi (450D), 50D and 40D. Canon EF 70-200mm f/2.8 IS USM and Canon EF-200mm f/4L IS USM were selected as the best telephoto lenses.
Ultra wide angle lenses might not be used as much as the above two lenses for most people unless you shoot landscape photography mostly. Canon 10-22mm f/3.5-4.5 USM is a great wide angle lens for Canon APS-C cameras such as Canon T1i and 50D. Comparing with the third party lenses such as Sigma 10-20mm f/4-5.6 and Tokina 12-24mm f/4, Canon 10-22mm has accurate focus, full time manual focus and less CA. But Canon 10-22mm is expensive ($749) even after the rebate. If money is not an issue, buy Canon 10-22mm.
Otherwise consider the options below:
Two better third party wide angle lenses have better value: Sigma 10-20 mm f/4-5.6 and Tokina 12-24mm f/4. Tokina 12-24mm is sharper, but it has some CA and not wide enough for some people since the extra 2 mm makes a lot of difference. If you shoot in Raw the CA can be fixed easily in Adobe camera Raw.
If you prefer a wider angle with good value, Sigma 10-20 might be the one for you. But keep in mind that Sigma lenses are built by reverse engineering and might not work on a new camera body.