Squirrelmail: http://www.squirrelmail.org

Horde/Imp: http://www.horde.org

Basilix: http://www.basilix.org

PhpGroupware: http://www.phpgroupware.org

Apache: http://www.apache.org

Php: http://www.php.net

Imap (uw): http://www.washington.edu/imap