Boolean Rithmomachia - Zillions of Games - A variation in which arithmetic/geometric/harmonic progressions are replaced by one number being the output of a boolean function of two other numbers, and the board is three-dimensional instead of two-dimensional. Implemented in the Zillions of Games framework.
Game Report: Rhythmomachy Basics - Description of the common elements of the many variations on the rules, based on Fulke's book. Does not cover moves, captures, or victories, where the rules vary.
Game Report: 16th Century Rhythmomachy - A reconstruction of one 16th-century variation, which is less common, but claimed to be more consistent and easier to learn and play than most versions.
The Philosopher's Game -- Rythmomachy - A transcription of a 1563 English translation by William Fulke of Boissiere's 1554/56 French description of the game.
Rithmomachia - Illustrated description of the rules of the game.