bughouse is of course an ever popular variant
antichess is fun on an amature level but not worth investing time in too many forcing lines (for example i know that 1.e4 is a forced win for black).
atomic chess (as played on itsyourturn.com) is a lot of fun
i like darkchess (where you can only see your pieces and the squares they could move too.)
team chess (players on a team alternate moves without talking)