I'll put in a vote for the 70-300 IS USM non-L, even though I hated mine. If you treat it as a 70-200 IS USM and compare it to the 70-200 f/4 L non-IS, then it's just that tiny bit softer, has vari-aperture, but has IS which can help you out in a pinch, and is cheaper. It's also smaller and lighter, and doesn't "stand out" like a White lens does (depends on where you're going to use it, of course).
Just don't ever ever shoot it at 300mm. I've heard it's a lot better at 300mm than others (like the 75-300), but it was still nowhere near the standard I wanted (maybe I got a dud, or I have higher standards than others?). And the rotating front element can be annoying, but if you use a CPL, just hold the edge to keep it facing the correct way as you focus, tricky but I got used to it.

But in the end I got sick of soft 300mm shots (I only ever really used it at 300mm anyway), I sold it to my sister and bought myself the 70-300L, and couldn't be happier. Yes it's more than twice the price of the 70-200 f/4L non-IS, and a bit more than the 70-200 f/4 IS, but you get the extra reach and it's a nice compact weather-sealed tank (I didn't mind losing the aperture in exchange for a very-usable 300mm). I can walk around all day with it on, I've just been shooting some racing at the Tour Down Under with it, works like a charm (I even got asked by security "what agency are you with?" when i tried to get around a barrier to get shots. I should have made up a newspaper I worked for and gotten past him...)