I recently posted a link to the Chess Tactics Server at our Middlegame links page. Hat tip to David T of Coolmind.net, who calls it "The equivalent of electronic crack."
The Chess Tactics Server is a wonderful test site with short game-like problems to solve under blitz conditions. Register or sign in as a guest (I recommend you try it as a guest first before risking rating points). You are presented with a position where you are playing either Black or White. Your opponent makes his move and you must find the right counter-move or two-to-three-move combination before time runs out. As soon as you solve it, the next position loads. Quite an addictive rush. Maintained by the German club Schachgemeinschaft Hermsdorf. Excellent training before your next serious or speed game. Beware of mouse-slips, though: it's touch-move!
Also of interest is Chess Position Trainer, which I will have to add.