Coffeerooms: Lost
General show and episode discussion.
http://www.coffeerooms.com/forums/tv/lost/index.html

Long Lost List
Show, cast, character, episode, and theory discussion. [Requires registration.]
http://longlostlist.kazorum.com

Lost Boards
Topic threads include news, episodes, characters, fan art, and fan fiction. [Registration required.]
http://lost-boards.net/

Lost Community
Show, cast, character, and episode discussion.
http://www.lostcommunity.com

Lost Fan UK
Discussion of plot, characters, and speculation.
http://www.lostfan.co.uk

Lost Forum
Discussion of episodes, characters, and relationships.
http://lost-forum.com

Lost Message Board
General show discussion.
http://www.fanforum.com/forumdisplay.php?s=&forumid=178

Lost News and Images
LiveJournal community.
http://www.livejournal.com/community/castaways/

Lost Paradise
Discussion of characters, episodes, and spoilers.
http://paradiselost.proboards24.com

Lost: The Show
LiveJournal community.
http://www.livejournal.com/community/lost_tv/

LostTalk.net
Discussion forums on the show, episodes, characters and theories.
http://www.losttalk.net

Media Blvd: Lost
General show discussion.
http://www.mediablvd.com/forums/viewforum.php?f=227