24 Forum
Topic threads include plot speculations, fan fiction, DVDs, and the character of Jack Bauer.
http://forums.prospero.com/fox24/start

24 Hours
Discussion of episodes, characters, music, and fan fiction.
http://s3.invisionfree.com/24_Hours/

Delphi Forum - 24
Discussion of plot, news, and spoilers.
http://forums.delphiforums.com/24plot

24Natic
Message board for fans of the show, with rules to prevent U.K. viewers from being spoiled on future episodes.
http://s7.invisionfree.com/24natic

Pure-24.com
Forum which includes discussions about episodes, news, and characters.
http://www.pure-24.com

Take 24
Message board with topic threads including gossip, interviews, and show seasons.
http://www.take24.co.uk/forums/

24/7: The UK's 24 Community
Message board based in the U.K.
http://groups.msn.com/247TheUKs24Community

Yahoo! Groups: 24_fans
Topics focus on the show and on Kiefer Sutherland. [Yahoo! registration required.]
http://tv.groups.yahoo.com/group/24_fans/

Yahoo! Groups: 24-FanFiction
Members post stories based on the series. [Yahoo! registration required.]
http://groups.yahoo.com/group/24-FanFiction/

Yahoo! Groups: fox_24
Discussion board for viewers. [Yahoo! registration required.]
http://tv.groups.yahoo.com/group/fox_24/

Yahoo! Groups: 24_Over_30
Discussion board with membership limited to people older than 30. Includes spoilers. [Yahoo! registration required.]
http://tv.groups.yahoo.com/group/24_Over_30/

Yahoo! Groups: Twenty_Four
Discussion of the series. [Yahoo! registration required.]
http://groups.yahoo.com/group/Twenty_Four/

Yahoo! Groups: 24UK
Message board. Spoilers for U.K. viewers discouraged. [Yahoo! registration required.]
http://tv.groups.yahoo.com/group/24UK/