I keep getting told that KingSolomon's is the most fish-heavy site on the net, but I have yet to try it myself. It comes very highly recommended by some of our regulars, though.
Absolute had a lot of god-awful players when I played there. FullTilt has a few, but typically just one or two really poor ones per table at $1/$2 limit.
Depends what limits and games you play, too, though.
The standard fish - the calling station - is the most commonly known species, but there are other variations that are just as easy (if not easier) to beat, if you can just spot them:
The weak/tight. If he raises preflop, it's with QQ or better. When he raises preflop, just fold, unless you're holding AA. There's no point in tangling with him, because if you have KK he has either QQ or AA. And being so passive, he won't pay you off enough the times he has QQ to offset the money you lose when he has AA. So I fold, unless there's more people in the pot (who provide "dead money" that weak/tight and I can share).
The loose preflop/tight post-flop. Not to be confused with your standard fish, these players won't go all the way to the river with nothing but third pair. They "fit or fold" the flop. With these players, you need to be aggressive pre-flop, because they won't stick around to pay you off if they don't flop something they like. And 70% of the time, they won't, thus raise strong hands preflop. If you can isolate them, raise anything preflop and bet the flop. They'll fold more often than you'd think.
The "I'm a big bad bluffer" variety. This is a somewhat loose player who now and again takes chances and goes berserk with nothing but 8-high. It often works. Pick your spot when you have a good hand, and let him get stung.
But my favorite is still the calling station. I bet, he calls. Sometimes he calls down with third pair and hits a second pair on the river, and I lose. It sucks, but the other 10 times he's done the same thing, he just paid me off. Love 'em!