GBuffy will poll multiple mailboxes for new mail. It will list the number of new messages in each mailbox you configure. It will also highlight the mailboxes that have new mail, and you can display the message headers by pressing mouse button one on a mailbox. GBuffy is currently capable of watching MBOX, MMDF, Maildir and MH Folders. This version also supports IMAP4rev1 and NNTP with XOVER mailboxes, as well as displaying X-Face headers.
tkbiff allows arbitrary commands to be executed upon mail reception. If you like programs such as xbiff and xbiff++ but wish they were more flexible, then you'll like tkbiff. Unlike other biffs, tkbiff is fully customizable. tkbiff also doesn't waste your valuable screen space with icons; instead, it shows you the mail itself. It supports UNIX, Mac, and Windows, IMAP, POP, and UNIX-style mail files, and SSL and APOP.
VAMP (Very Attractive Mail Program) is an extremely easy-to-configure, usable, Web-based email client written in PHP3 (and compatible with PHP4). It is for use with POP3 servers, and it also supports "Virtual Domains", encryption (under Linux/PHP3), attachments, contact list, a signature database, an easy-to-use setup tool, and more.