I've been jonesing for a zoom lens since I picked up my Canon XT back in June. The kit lens is not too bad (aside from some chromatic aberration), but a longer lens would be great. I had been eyeing the Canon 55-200 EF lens, at around $300 bucks at the local Black's, but after reading some reviews, I want to stay far far away from that lens and its questionable build quality.
After a bit of poking around on The Camera Store's site, I think I've found a much better alternative. A Sigma 70-300mm f4-5.6 APO Macro. There's no image stabilization, so I'll have to use a tripod when in the 200-300mm range, but at $320CDN, it's a much better deal. And reviews suggest a much sturdier build quality.
Poking around some sample images shot with that lens, I'm pretty happy with it. Sure, it might not stack up against a $1500 lens with image stabilization and the works, but it's cheap enough to be able to pick it up without many regrets.
Unfortunately, I think the lens is too slow for much indoor work. I don't think it's a suitable candidate for photographing workshops here at the TLC. But as an outdoor lens, it looks pretty darned good.
Update: On recommendation from Raffaella, I think I'll hold out for the Canon 28-135mm f.3.5-5.6 IS USM. It's a little more spendy (just under $600 CDN), and not quite as long, but the Sigma may be too long, and the build quality won't be quite as good as this. And the image stabilization would help when shooting at the 135mm end of the lens. Now to go return some more empty bottles, and look under the cushions on my couch...