If for zoom you mean ZOOM, I mean from 50mm to 200 or 300 mm or so, than I advice the Canon EF 70-300 IS which I have and it's very good, almost a L lens. And the IS sometimes is very useful...
If on the contrary you mean a walkaround lens, there is the cheap Sigma 17-70 2.8-4.5 wich is not bad (I've been using it since about six months) but not so sharp and not very fast (but for the price you can expect more

) or, a little more expensive, the good Tamron 17-50 2.8 that is sharper than the sigma and 2.8 across the entire range (but the sigma is also a macro lens, and the tamron not...) or the Sigma 17-50 2.8 which is also macro...
Last but not least the canon EF-S 17-55 2.8 IS seems, according to the reviews, very good, but also quite expensive (about 1000$ or so...) and a little too short (55mm) but it depends on which kind of photos you normally take
There is also the canon 17-85 IS but it's not fast at all (4-5.6) and, for the price, has, for me, too much problems (vignetting, not an very high sharpeness, strong chromatic abberration etc.). But some people say it's a good lens, so it's up to you
Anyway on the
http://www.photozone.de there are good reviews...
Bye
CarloF
Canon 350D
Sigma 17-70
Canon EF-S 60mm macro
Canon EF 70-300 IS