Free Mail Server & Webmail: XMail Server Lite
XMail Server Lite is an open source & free mail server with an Ajaxed webmail bundled to it.
It is compatible with Window & Linux OSs with 2 different installers provided.
The server is capable of managing SMTP/ESMTP & POP3 protocols with web-based administration panel.
It supports unlimited domains/users including virtual domains & aliases.
XMail Lite also offers anti-virus integration, bayesian filters (for spam protection) & a logging functionality of all the activities.
The Webmail Lite can display folders/nested folders, has an address book, multilingual & much more.
Tip: The webmail can be downloaded & installed as a standalone product too (from here).