I suppose it depends on the type of stuff you like to shoot. I was often trying to capture the immenseness and grandeur of things, and would have loved a fisheye. A lot of those big ancient buildings and such would look really ccool with the distortion and it would have saved me a lot of time pasting images together. I've only played with Toshi's sigma tho, so I can't really suggest a specific model.narlus said:phoben, sweet shots (at least the ones toshi cross-linked). i'll be in Rome in a few months...what lens should i buy to maximize my photographic joy? at this point i've only got the 18-55mm kit lens, and a 50mm prime for my rebel xt.
The other thing that would be really helpful is a lens with IS type I, as there are lots of places where the light is not so good, but a flash is either useless or not allowed. I was using the canon 17-85 f/4-5.6 IS USM exclusively while I was there, and found myself regularly shooting ISO 800 at speeds of 1/15 or so. With IS this is totally reasonable, but without it you're just gonig to get garbage. If you want to upgrade your zoom, this lens is a pretty awesome all-purpose lens and it almost never comes off my camera. I find it makes especially nice images with a polarizer, but the 2-stop hit you take limits where you can use it effectively.
If it weren't for the light issue, I'd suggest the 70-200 f/4L USM as a nice addition too, but unless you plan on using it outside exclusively, you're going to have to hump around a tripod to make pictures that don't suck. If you have the $$, the 70-200 f/2.8L IS USM is the perfect range with all the features you need, but man is it pricey.
finally, get hoods for your lenses. I had a lot of trouble with nasty flare while shooting outside. I don't know if the smog had some effect, but it seemed that every time I had sky in a picture I had huge flare splotches that could have been avoided if I simply had a hood.