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