T1i and a 3rd lens?

I also have a T1i (500D) and a set of lenses. I bought the camera with the 18-55 kit lens and also got a 55-250 at the same time. Those have in practice both been replaced by a Sigma 17-70 f2.8-4 and an EF 70-200 IS f4, and those two lenses are the ones that spend by far the most time on the camera - I would say that 80% or more of my shots are taken with the 17-70, and 10% with the 70-200.

Then I've also got some other lenses which I bought because I thought I'd use them, or because other people said they were cool, or for some other reason. These include an EF-S 10-22 and an EF 35 f2. Then I also still have, from film photography days, an EF 28-105 f3.5-4.5, and an EF 50 f2.5. Of those, I do occasionally use the 50mm, and very occasionally the 35mm, but the others hardly ever get on the camera. That includes the 10-22 which I originally thought I would use a lot. Not so; it turns out that I'm not a wide-angle photographer and the wide limit of the Sigma (17mm - about 26/27mm in full-frame equivalence) is as wide as I feel comfortable with.

I spent about 20 years using the 28-105 on a series of film cameras and I think I got very used to that range of focal lengths. It's interesting that the 17-70 covers about the same field of vision on an APS-C camera as the older lens did on a film camera. I've never really wanted 'wider' (which makes me wonder why on earth I bought the 10-22); I do sometimes want 'longer' and the 70-200 meets that need.

Looking at your lenses, while I know the 17-55 is an excellent lens as regards IQ (better than my Sigma), I would ask - do you find it's long enough? If you're otherwise happy with the lens selection you've got, perhaps filling in that gap between 55mm and 70mm is the only thing missing?

Oh, one other thing - I do intend to get a 1.4x converter at some time. Perhaps that might be for you as well?