For APS-C the Tamron 18-270 provides a great range of focal lengths. It also has stabilisation (Tamron calls it 'VC')
I own it, very happy with it. The IQ isn't as good as the 17-50 f/2.8 but you can't beat the versatility. I used it as a primary lens on a 4 week holiday, and was very happy with it.
It is a little slow, so it works best when there's plenty of light (thought the 7D has better ISO than my 405D). Also the zoom mechanism is a bit loose, except in the middle where it's tight. You do need to use the zoom lock to stop it sliding out.