IMP is the Internet Messaging Program. It is written in PHP and provides webmail access to IMAP and POP3 accounts.
There are several current branches of IMP. IMP 4.1.x is the current production version of the stable branch. It requires PHP 4.3.0 and version 3.x of the Horde framework. It adds new features like crypting support, flexible charset handling, virtual folders for saved searches, a WYSIWYG editor for creating HTML messages, improved MIME message handling, and many more.
IMP 3.2.8 is the latest production version of the previous stable branch. It requires PHP 4.1.0 and version 2.0 of the Horde framework. It adds advanced features such as searching multiple mailboxes, identities, and a hierarchical mailbox navigator, as well as a cleaner, redesigned user interface. As with IMP 2.2 before it, it passes the "MIME Torture Test" that UW makes available.
Feel free to view (or add) documentation on IMP on the IMP Wiki Page.