I currently use only two Canon lenses for my 20D; one is a 50mm f1.8, the other is 28mm f2.8 - both produce very nice pictures, and together cover an impressively wide range of areas to shoot in. The only thing I'm lacking is a proper telephoto objective, which I'm currently thinking of filling with a 75-300mm zoom lens.
You just have to keep in mind, you must multiply the focal length with about 1.6 or so, since your sensor is smaller than a 35mm film. Thus, a 28mm covers all kinds of landscape pretty okay, and the 50mm is fair enough for nature photography, and spot-on for portraits. For shooting birds, animals or other things you can't get close to, though; you'll need a bigger one than 50mm anyway. But the beauty is that both these objectives are physically tiny, really light, and got a really nice aperture since they're not zoom-lenses. Plus they're really cheap.
(If anyone wonders what happened to the zoom-lenses I used before, which I miss a lot even if I have these two allrounders, I had to turn them in with the D60

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