VMware Zimbra is an enterprise-class email, calendar and collaboration solution, built for the cloud, both public and private. With a redesigned browser-based interface, Zimbra offers the most innovative messaging experience available today, connecting end users to the information and activity in their personal clouds.
via Zimbra offers Open Source email server software and shared calendar for Linux and the Mac..
The slashdot comment from whence I found out about this recommended *not* to use this service. Interesting nonetheless.