I must admit I'm not much into board games (except for chess) so backgammon is relatively new to me. And I was kinda surprised it's really popular, coz if it's not, would there even be a backgammon online version? I think not! So far, the online backgammon I've tried is the one in Gammon Fortune. Just download their software and get ready to play backgammon against players worldwide. For those who are not familiar with backgammon, here's a brief description from Wikipedia:
Backgammon is a board game for two players in which checkers (also called pieces, stones, men, counters, pawns or chips) are moved after each roll of the dice. A player wins by removing all of his checkers from the board. As the playing time for each individual game is short, it is often played in matches, where victory is awarded to the first player to reach a certain number of points.