Chess Tactics Server at http://chess.emrald.net/
Its a pretty good site to check out. At first I was frustrated at the 3 to 7 second window you get to solve a problem, but after a while, you start to get better at finding tactical shots.
They have over 20,000 problems. I think alot of the problems came from actual chess games. In any event, the problems span a huge range of motifs… sometimes you get Mate in X, but often you just get a material advantage without a forced mate. Its good, since you quickly realise that alot of OTB tactics are not Mate in X.
I make a distinction, becuase I own a chess “tactics” book, in which every problem is Mate in X… usually by sacrificing something. Although in a perfect world, every game could be won by some cool queen or rook sacrifice, I prefer the Chess Tactics Server with dishing up problems that are more likely to occure in a real game… that is, a good mix of “mate in X” and “win material” problems.