Coffeerooms: ER
Bulletin board, character information, and cast lists.
http://www.w3pg.com/tv/er/

ER Boards
Includes episode discussions and fan fiction.
http://carolhathaway.proboards18.com/index.cgi?action=login2

ER UK
Message board for ER fans mainly in the UK but others are welcome. Boards for Channel 4, E4 and US viewers and a password protected board for ER spoilers.
http://eruk.proboards27.com/

Yahoo! Groups: ER
Message group for discussion of storylines, cast members, and spoilers. [Yahoo! registration required.]
http://tv.groups.yahoo.com/group/er/

Yahoo! Groups: ER- Explained
For discussion of medical issues related to recent episodes. [Yahoo! registration required.]
http://tv.groups.yahoo.com/group/ER-explained/

Yahoo! Groups: ER_Junkies
Forum for addicted viewers. [Yahoo! registration required.]
http://tv.groups.yahoo.com/group/ER_junkies/