Triple Yahtzee

Rules for a Yahtzee variant.
http://www.centralconnector.com/GAMES/tripleyahtzee.html

Rules of Yahtzee

Includes all the rules of standard play.
http://grail.sourceforge.net/demo/yahtzee/rules.html

Kismet

Rules for a variant of Yahtzee that uses different dice.
http://www.centralconnector.com/GAMES/KISMET.htm

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.
http://www.gypsyware.com/

Free Comp Stuff

A site offering a free Yahtzee game for Windows and links so you can make your own games.
http://www.geocities.com/freecompstuff