If you need a wide angle, the Tokina 11-16 f/2.8 stands alone in bang-for buck on APS-c. They've just announced a mkII version that should be out this summer.
If you've got a dedicated wide-angle for your crop camera then a general-use lens designed for FF cameras like the 24-105L IS is less of a limitation on the wide side (as it becomes a "normal-to-medium tele" lens on crop bodies).
You almost certainly want some kind of prime as well though. The 85mm f/1.8 USM is a really great option that won't break the bank. I use it for portraits, landscapes, concerts, low-light, etc etc etc. Good lenses that are light & fit in a coat pocket have a certain advantage over the big L zooms sometimes...
Personally I use a Voigtländer 20mm f/3.5 Color Skopar as the "street lens" on my 40D. Small, light, inconspicuous, and it's a very rare example of an electronically coupled EF mount pancake lens (the other example being the Voigt 40mm). I usually just set it to hyperfocal at f/4 & forget about having to focus :-).