Insaniquarium
This wildly addictive free online game challenges you to grow and raise a collection of fish (and protect them from enemies). Insaniquarium combines elements of arcade fun, strategy and virtual pets. There's also a "competition mode" that lets you challenge other players. (Note: this game is optimized for Internet Explorer).

Phat games
A fun, free online gaming site that features such popular titles as multiplayer backgammon and chess, as well as over 30 single-player games. Most of the games are Java-based (which means that you can jump right in and play, without downloading anything).

MSN gaming zone
Looking for some real, live competition in free online gaming? If so, this site is hard to beat. Microsoft's cutting-edge gaming center is the biggest resource of its type on the Web, with over 18 million registered users. Membership is free. Games here include classics such as backgammon, bridge, checkers, chess, cribbage, go, hearts, reversi and spades.