JohnWiegley has written a chess mode. It plays against crafty and gnuchess. You can also play on Interenet Chess Servers, like FICS.
A possible computer oponent if you are not playing against a fellow human.
Note the licence: Crafty is freeware and is copyrighted by Bob Hyatt. Parts of the Macintosh port are copyrighted by Lloyd Lim and by Metrowerks. You can do anything you want with Crafty, as long as it isn’t for any commercial purpose.
MarkOakden has written an emacs mode for communication with internet chess servers (FICS http://www.freechess.org , ICC http://www.chessclub.com, etc.) It handles the text part of the session, so unless you like playing in ASCII, you’ll still need a board such as XBoard ( http://www.tim-mann.org/chess )
The chess.el project also contains an unrelated implementation of ICS communication, called chess-ics.el. To startup chess.el against a ICS server, use C-u M-x chess RET ics RET