http://www.tim-mann.org/chess.html
Tim Mann is the author of Winboard (a free graphical interface to allow computer programs to play against each other), also information on: GNU Chess, XBoard, WinBoard, Crafty, Internet Chess Servers, Zippy, Chess Web Sites, Chess FTP Sites, and more.

http://www.playwitharena.com/
Arena is another free graphical interface for computer chess programs. Arena supports the winboard and UCI protocols and has a lot of extra features.

http://www.wbec-ridderkerk.nl/
This is probably the most famous tournament for chess engines.

http://www.freechess.org/
FICS is a Free Internet Chess Server.

http://www.chesscube.com/
Another Free Internet Chess Server with a Java interface. Not so many opponents, but at least they're all human!

http://gameknot.com/
For those who only have a few minutes per day to play online chess.

http://www.chessville.com/
Chessville has a lot of chess related stuff. You can also subscribe to their newsletter.

http://www.chesscenter.com/twic/twic.html
The week in chess is a chess news site.

http://www.limunltd.com/crafty/
The famous free chess program, Crafty. The source code is also available for download.

http://www.cis.uab.edu/info/faculty/hyatt/pubs.html
Technical papers by the creator of Crafty, Robert Hyatt.

http://members.home.nl/matador/chess840.htm
The author of the commercial program, REBEL explains some of the programming techniques used in REBEL - very interesting!

http://supertech.lcs.mit.edu/~heinz/dt/
The home page of DarkThought. Some interesting topics on how DarkThought plays chess.

http://www.seanet.com/~brucemo/index.htm
The home page of Bruce Moreland (author of Gerbil) Contains information on computer chess programming on the Programming topics page.