IMP
IMP allows universal, web-based access to IMAP and POP3 servers and provides an addressbook, LDAP directory searches, full support for sending and recieving MIME attachments, and many other features normally only found in desktop mail clients. IMP is actively developed under the GPL.

W3Mail
A web-based POP3 e-mail client written in perl used with a HTTP server.

WebMail
A standalone Java-based HTTP server www-frontend to IMAP/POP3 mailboxes.

twig -- The Web Information Gateway
A PHP-based intranet/groupware tool with scheduling, newsgroups and mail (via IMAP) capabilities.