Hi Gina,
This is asked and answered several times a day and really hasn't changed recently. Lenses used on any of the APS-C crop cameras (currently T3i, T2i, 7D and 60D) will behave pretty much the exact same way on your XTi. Your camera can be used with any EF or EF-S mount lens.
First decide if you want wider than what you've got now, or if you are wanting to upgrade the focal lengths you have now in some ways.
Your 18-55 is a moderate wide to moderate tele lens. The "premium" upgrade for that is the Canon 17-55/2.8 IS. A slightly wider and longer and popular option is the Canon 15-85, which is kept reasonably compact by not being a fast f2.8 lens. Both of these have USM and IS. These sell for $750-1000. A popular "budget" lens that has good image quality is the Tamron 17-50/2.8 non-VC, about $400 or so, though it's autofocus isn't terribly quick. There are several other, similar "standard" zooms at various price points. They would essentially be a replacement for your 18-55mm. Don't expect a whole lot of improvement in image quality... rather they will have other premium features that might make them a worthwhile upgrade choice.
A true wide angle or "ultra" wide angle would expand upon what you have now and include 15mm and wider focal lengths. A lot of landscape photographers enjoy using expansive lenses such as these:
Canon 10-22
Tokina 12-24/4
Tokina 11-16/2.8
Sigma 8-16
Sigma 10-20/3.5
Sigma 10-20/3.5-5.6
Sigma 12-24 (pricey, it's a full frame capable lens)
Tamron 10-24
These are not fisheye lenses (there are a bunch of those avail. too, but they have strong distortion effects). The above range in price from a little under $500 (one of the Sigma 10-20 and the Tamron) to over $1000 (Canon 10-22).
You can find tons of discussion about each of theses here on POTN. There is also the the Lens Sample archive sub-forum attached to this one, where each post is dedicated to people sharing images made with any particular lens. Search it for any you are considering.
Long story short... I compared a few of the above and bought the Tokina 12-24/4 a few years ago... still happy with and using it. I consider it about the best value/quality combo. The overall best image quality I think is the Canon, but it's a lot more expensive and the Tokina seems quite a bit better built.