DeskNow is a fully featured and easy to use mail and collaboration server that can handle all the communication and collaboration needs of your company. DeskNow Lite is a free version with limited features, ideal for home users and zero-budget organizations.

It features a full mail server, webmail, webcalendars, document management and sharing, discussion groups, shared addressbooks, web publishing, spam filtering and many other collaboration tools

DeskNow Lite can be installed on Windows, Linux, FreeBSD, Solaris, Mac OS X, etc. For more information, refer to http://www.desknow.com/desknowmc/features.html.

The freeware DeskNow Lite can be downloaded here.
User Manual: http://www.desknow.com/downloads/DeskNow%20-%20User%20Manual.pdf

The free Lite version can be upgraded to the Professional version (commercial), which adds secure instant messaging, access from Palm, PocketPC and WAP devices, access to remote files as a network drive, and more.