Free Webmail Software: Hastymail
Hastymail is a free IMAP webmail software (open source) built with PHP.
The application is lightweight, has a clean interface & compatible with all major browsers including mobile ones.
With its plugin support, new features can be added. Some available plugins are: calendar, filtering, RSS/Atom.
Hastymail can be customized via CSS & has a PHP based template system.
It has Vcard based contact list with multiple import/export ability & many other features.