What are some fun or creative chess variants that you have played?

Currently looking for some fun ones to try out after playing hexagonal chess

Spartan Chess ( http://www.spartanchessonline.com ). Similar in complexity to normal Chess, (not so horribly tactically complex as Capablanca or Seirawan Chess), with significantly smaller draw rate and many interesting end-games:

A more modest variant is Knightmate, where the King moves as Knight and vice versa, and you have to checkmate the royal Knight.

Try bughouse.