Each round, all players are shown a funny (or supposedly funny) picture, and must type the funniest caption they can within 45 seconds. Once time is up, all the other players' captions will appear, and everyone must vote one which they think is the best. There are 7 rounds of this. In the first 3 rounds, every vote a player recieves is worth one point. In the next 3, each vote is worth 2 points. In the final round, a special rule is added (usually to have the caption fit an acronym or be in a certain style), the time limit is extended, and each vote is worth 3 points. Whoever has the most points at the end wins.
WTF is free, and does not require registration.
The game has moved from its original host to a new site, Side-Quest Games.
MakeYourChance is extremely good - watch out!