If you are sticking with your Rebel XT (or even upgrading to another cropped sensor camera body), then I highly recommend the Tamron 17-50mm f2.8 lens. It's a great step up from the kit lens and is very versatile just like your kit lens in the same range. It's a great little all purpose lens.
It's relatively fast with f2.8 across its entire focal range unlike the canon kit lens that is variable aperture across its range. If you want a bit more bling it happens to come in a Vibration Compensation model (for more money of course). I bought this lens when I had a Rebel a few years ago and still use it with my Canon 40D. I recommend it to all of my friends who also love this lens. I love this lens so much I bought it's larger brother, the Tamron 28-75 f2.8 that's meant for full frame camera bodies like my 6D
