IMP 

http://www.horde.org/imp/

IMP is (Internet Messaging Program) webmail application written in PHP and based on Horde framework.

IMP provides webmail access to any IMAP or POP3 mailbox, and handles internet standard MIME attachments, user defined filters, preferences, and more. IMP was the first Horde application, and in some respects Horde grew out of it.

IMP have support for many interesting 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.