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.
(gem) Når du rykker i et spil, kan du vælge hvilket spil du går videre til ved at vælge i listen ved siden af træk knappen. (pauloaguia) (vis alle tips)