Triple Yahtzee
Rules for a Yahtzee variant.

Kismet
Rules for a variant of Yahtzee that uses different dice.

Killer Dice from Gypsy King Software
Killer Dice is a Macintosh dice game based on the classic Yahtzee. It includes single, double and triple games. From Gypsy King Software.

Free Comp Stuff
A site offering a free Yahtzee game for Windows and links so you can make your own games.

Rules of Yahtzee
Includes all the rules of standard play.