Monochrom's Massive Multiplayer Thumb-Wrestling
   Explains the basics as well as multi-player variations. Includes photographs.
   http://www.monochrom.at/daumen/netzwerk-eng.htm

Samurai Thumbs
   Describes how to play this game using helmeted thumb wrestlers as warriors. Includes some variations.
   http://www.deepfun.com/thumb.htm

Thumb Wrestling
   Describes how to thumb wrestle, with a snake in the grass variation.
   http://www.gameskidsplay.net/games/other_games/thumb_wrestling.htm

Wikipedia - Thumb Wrestling
   Description of how the game is played.
   http://en.wikipedia.org/wiki/Thumb_Wrestling