Best Places to Play Chess Online Free

ChessOK.com

1. ChessOK.com. You can play online in your browser, or download their powerful software for more features. Mostly Russian players. You can use their software to automatically check your game for mistakes once it is finished. Also, I really like the colors and design of their boards and pieces. It also allows you to store your games.
2. Owl21. Relatively new. Continually improving. Great interface and sounds. There are many Spanish speakers here.
3. Internet Chess Club (not free: about $60 US dollars a year) Definitely the best chess club on the Internet. Features include ability to watch some of the strongest players in the world play live, listen to chess-related radio programs, and study from a huge collection of chess training videos. Expensive, but worth it if you are serious about improving your chess. 7-day free trial available twice a year.
4. GameKnot.  Offers both very long games (e.g. one move/day or longer) and blitz.  Good interface.  Blitz doesn't give any control over who your opponent is.
   
   
   Others to Consider:
   

1. The Free Internet Chess Server. Started by ICC members unhappy at being charged money, FICS is not very pretty but it has a large community and a powerful specialized browser (client) to play chess in. It also allows you to instantly run a computer check after you play a game to see what your mistakes were. It also allows you to store your games. Its main drawback is the complicated and confusing interface.
2. Chess.com. A relatively new chess community that has many great discussions and features. However, there are better places to actually play chess.
3. Playchess.com by the makers of the powerful (and expensive!) Chessbase training products. They offer the ability to use their client free. The colors of the interface are excellent, but like their other products, using their features is confusing when it shouldn't be. Mostly European players. Has in interesting feature that shows where in the world the online players are.
4. Yahoo chess is popular but is probably the worst place to play on the Internet. There are many people cheating with software, and players there tend to be childish. If you manage to win a game, it is unlikely your opponent will accept another game with you, instead prefering to kick you from the chess table. Beware of abusive language as well.  One advantage is that you can email your moves to your yahoo email account.  The other websites I recommended above all do that as well though.