From http://horde.org/imp/
IMP is the Internet Messaging Program. It is written in PHP and provides webmail access to IMAP and POP3 accounts.
IMP is an incredibly useful program. It supports many features of IMAP, such as folders, but alas does not do Fcc's. If you don't mind the rather common use of JavaScript throughout an IMP-enabled site, it makes e-mail a very simple task when IMAP, SSH and Telnet ports are blocked by a firewall. It supports SSL, both when acting as a client to the IMAP server and a server to the user.