A Mind Game, origin unknown, that is well suited to chat and a medium number of players. Each round, each player privately submits one word to the Game Moderator that has not been submitted in any previous round. Then, the mod reveals all the submitted words, and any words submitted by exactly two players (no more, no less) earn their submitters a point. First to N points wins.
No game-related discussion outside of the words themselves is allowed, so the attempts of the players to be obvious enough in their word selections so that someone else follows along while not so obvious that anyone else does can be quite interesting.