Interactive mathematics miscellany and puzzles, including 75 proofs of the Pythagorean Theorem, an interactive column using Java applets, and eye-opening demonstrations. (Actually, much more.)
Tetrod is a jigsaw puzzle and a four-sided domino game mixed together. -- Java puzzle game; choose 3x4, 4x4, 5x4, or 5x5.
Flea Circus! It's just like Lemmings! Only tinier! And faster! And you can only make three moves: Block, ramp, and other ramp! (java)
The Flowering Nose in Slugland adventure game. As a goblin with a flower for a nose, your ultimate goal is to find the lost sprout. Defeat enemies by throwing flowers at them; power up with hearts; teleport from level to level with such esoteric trinkets as donuts and cherries. (Java.)
Friday Java Fun - Junk's Hanoi, a cool puzzle for this lovely Festivus.
this childs game (java applet) is so complex you could use it to construct a computer. the original paper (postscript), is heavy going - if you're not a compter science student but would like to understand more try this wonderful book by one of the authors (example pictures).