Wide angle lens

It has a good working range, image stabilization, and light weight. It is a bit soft, but achieves pretty good results for its cost.

The 17-85mm IS lens is not a small lens, just a bit lighter than your 28-135mm IS lens (16.1 oz vs 18.9 oz). The build and optical quality are similar, except different focal length. Buying both will have significant overlap of focal length.
The EF-S series also a 10-22mm wide angle lens, but this is not to be used for any close-up portraits unless you want to get distorted funny faces with a big nose.
And of course there is the econo EF-S 18-55mm kit lens, which is not a problem when used outdoor in good day light. But in lower light condition, it lacks the IS and USM.
The best Canon wide angle zoom lens is the EF 16-35 mm f/2.8L II lens, followed by the EF 17-50 mm f/4L. Both of these are similar in size to the 28-135 mm IS and 17-85 mm IS lenses respectively. But of course they are much more expensive.
There are not a lot of wide angle prime lenses for the Rebel XTi. Most of them are for full frame bodies. After you multiply the 1.6x conversion factor, many do not function as wide angle lenses with the XTi. There is the expensive 14mm f/2.8L, and the much cheaper 20mm f/2.8 lens (but not really wide on the XTi).
If you don't need to be absolutely loyal to Canon, Tamron has a 17-50 mm f/2.8 lens which is quite nice and not expensive. It is a little bit lighter than the 17-85 mm IS lens, at 15.2 oz. It does not have IS, but it is a faster zoom lens with a fixed f/2.8 aperture which will compensate for its lack of IS. It has received decent reviews. It has very good image quality and can even be used in indoor and low light because of its f/2.8 aperture.