POPFile is a powerful, cross platform email classification POP and NNTP proxy, that you can use to filter spam efficiently and categorize good mail automatically. You can Automatically Sort Web Emails with Free POPFile.   

The Free Web Mail Software SquirrelMail was developed by Nathan and Luke Ehresmanis and features a stable, feature-packed and a fairly configurable email server. To use the software you’ll need a Web Server running PHP, and an IMAP Server to connect to.

gnubiff is a Free Multi Account Email Notifier that checks for mail and displays headers when a new mail has arrived. This software supports various mail servers such as the pop3, apop, imap4, mh, qmail and mailfile. You can  Receive Desktop Alerts on Emails from Multiple Accounts using this software.