4 links
Monochrom's Massive Multiplayer Thumb-Wrestling
monochrom.at
Explains the basics as well as multi-player variations. Includes photographs.
Samurai Thumbs
deepfun.com
Describes how to play this game using helmeted thumb wrestlers as warriors. Includes some variations.
Thumb Wrestling
gameskidsplay.net
Describes how to thumb wrestle, with a snake in the grass variation.