Whilst everyone jumps in with the biggest most costly options, a lower cost option is the 70-300mm non L, which although isn't great in low light, works fine for me in terms of getting the occasional long shots - it has IS which helps, but for less than the price of the 70-200mm f4 and half the 70-200mm f4 IS, it's a good bargain and the picture quality is pretty good... Plus you get the extra reach to 300mm, I've paired mine up with a 15-85mm which is on my camera most of the time.
Wait, are you talking about the EF 70-300mm f/4-5.6 IS USM?
I've been sorely disappointed time after time by this lens; grainy and low-res compared to pretty much every other lens in my collection
We went up to Alex Bay last weekend and I have to say that the 70-300 shots were so grainy even on a 13" notebook that recomposing the shot and using the 24-105L seemed much more professional . . .
I'm definitely in the market for a higher quality telephoto.