One of the best was one from my high school days called "Doubles". You need 2 boards and 4 players - 2 teams of team - one black, one white. The object is the standard checkmate, but
1) When you take an opponent's piece, it is given to your teammate
2) Your teammate can put captured pieces anywhere on the board except that pawns can't be placed on the last row (so you have to promote your pieces the hard way)
3) It is blitz chess - 5 seconds or less per move
4) Friendly insults are mandatory.
5) If you tell your teammate "anything for a knight", don't bitch if they give up a queen for it.

Any other fun variations out in the SD world?