I would highly recommend the EF-S 17-55mm f/2.8 IS USM. I used this on my 350D and 7D for a couple of years before selling it on. My reason for getting rid of it was simply because I preferred primes to zooms. The optical quality of this lens is stunning, and should be an L series lens in many peoples opinions. It has aspherical elements and coatings like the L series, though the body of the lens is not metal like an L series and is not weather sealed. Otherwise it's great. The fast f/2.8 aperture through the whole zoom range is also fantastic, allowing you to get some great indoor or low-light photos at lower ISO's than would otherwise be necessary.

In full-frame terms, 17mm is equivalent to 27mm and 55 is equivalent to 88mm, so you're getting a 27-88mm lens (equivalent) with constant f/2.8 aperture AND image stabiliser too.

It should be noted that EF-S lenses are designed specifically for crop sensor cameras, and I read somewhere (perhaps here) that ON a crop-sensor body, an EF-S lens would be sharper than the equivalent EF lens, which is designed with full-frame sensors in mind. Obviously the downside of an EF-S lens is that should you decide to go full-frame in the future, you will not be able to use the EF-S lens on a full frame body.