It would be great to have a 2-player bughouse on this site. Please see http://www.chessvariants.org/multiplayer.dir/tandem.html "Bughouse per e-mail".

I think bughouse is the most popular of chess variants (if you don't count national variants like Xiangi or Shogi). It is usually played by 4 people, but I think 2-player version should be fun as well.