One of the best deals in the world of lenses is the Canon 18-55mm lens that's available in a kit with the Digital Rebel XT (350D). It's weighs only 6 ounces (190 grams) and takes sharp, saturated photos. Sigma has designed what I consider to be the perfect complement to this lens. Their 55-200mm DC Zoom weights only 11 ounces (310 grams) and extends the zooming range of the Rebel all the way to 200mm. When you consider that on the Rebel this 200mm focal length is actually 320mm because of the 1.6X magnification factor, you gain a lot of reach without adding much weight to your camera bag.
I've been toting the Rebel with the two lenses for a couple months testing to see if I could get by with just these two options for the variety of assignments I encounter. I have to say, this tandem works great and weighs almost nothing. Unfortunately, the filter ring size is different for each optic -- the Canon takes 58mm filters and the Sigma uses 55mm. I could go with a 55-58mm step-up ring and carry only one diameter of filter, but I've opted for packing both a 55mm and 58mm polarizer -- just seems more convenient in the long run.
The performance of the Sigma 55-200 is outstanding. The images are crisp and saturated. The lens doesn't have the silky smooth autofocusing that you get with Canon lenses, but the action is precise and not too loud. One thing that long-time Canon shooters will notice is that the zooming ring turns the opposite direction that Canon lenses rotate. This does take some getting used to, and is really the only drawback I encountered with this tandem.
You can buy the Sigma 55-200mm on Amazon for $125, and that includes a lens hood. The only other lens that I would include in my "basic on-the-go DSLR kit" would be the Canon 50mm f-1.8 optic. This gives you a lightweight (4.6 ounces / 130 grams) low-light capable lens without much additional bulk.