Free

Dolphin Dice
A Yahtzee game for Windows that features high scores comparisons with others from around the world.
http://www.johannorberg.se/dolphin/

Knuffel
Windows game based on 'Yahtzee' with a mailing list, chat page, and high scores list.
http://www.plenio.com/knuffel/

Yahtzee Master
For Windows and Linux, features multiplayer, a computer player and high scores.
http://jamesgart.com/yahtzeeonline/index.html