Any lens made to fit a Canon EOS EF mount will work with the Rebel XT. The kit lens that came with your camera is the Canon EF-S mount that is specifically designed for DSLRs with a smaller sensor (full sized being defined as the size of 35mm film). If you purchase EF-S lenses they will not work with full size sensor cameras or the film cameras (if you ever decide to buy a new one in the future). But they are typically lighter and smaller in size, which make them attractive.
The full size EF lens will also fit your camera and you will find many more options to pick from this in lens range, quality, and brands. One advantage of the EF lens size is that you are not shooting through the whole lens, but mainly shooting through the middle of the lens. Many lenses are rated or noted as having some distortion or softness around the edges until stopped down to a certain point, when mounted on an EF-S mount this is virtually eliminated.
Three of my favorite zoom lenses are (and I know they can be costly, but these are an investment in quality):
Canon 28-135mm f/3.5-5.6 IS USM
Canon 70-200mm f/4 L USM
Canon 24-105mm f/4 L IS USM
A really good wide zoom (potentially one of the best made) is the Canon EF-S 10-22mm f/3.5-4.5 USM ... it is fairly fast and very sharp. You can also look at adding some primes (fixed length) lenses to your collection. The advantage of a prime is that it will usually be very fast (f/2.0 or faster meaning you can take pictures in lower light without a flash) and they are quicker to focus.
You can check out these sites to get more information on specific lenses
Fred Miranda or
The Digital Picture
Mike