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 compatability with existing infrastructure and applications (both open source and proprietary). Zimbra is open source in order to leverage the collective talent of the greater community, and to ensure investment protection for Zimbra partners and end-users.

Very nicely put together AJAX collaboration suite. This looks like an open source replacement for Exchange! I wish we administrated this at work instead of Mirapoint's (less functional) equivalent. Watch the flash demo, then try the live demo.