From the Zimbra website:

"Zimbra is an open source server and client technology for next-generation enterprise messaging and collaboration. Zimbra delivers innovation for both the administrator and the end-user as well as compatibility with existing infrastructure and applications (both open source and proprietary)."

---

Zimbra's namesake Zimbra Collaboration Server (ZCS) has already won praise from eWeek, InfoWorld, and Forbes magazines. In addition, ZCS was voted "Best Enterprise Project" at the 2006 SourceForge Community Choice Awards, and won the SIIA Codie award for "Best Communication or Collaboration Solution" (link)

Please see their website for screenshots, or to try their hosted demo. Zimbra also has a Flash demo which you can view on their website. Additional documentation is available from Zimbra's website.

*Note that "Zimbra", the Zimbra logo, and "Zimlets" are trademarks of Zimbra Inc. ZiOffice is a Zimbra hosting provider, but is otherwise not affiliated with Zimbra Inc.