Microsoft has its Microsoft Exchange and it is really one of the best solutions for the office. But, Zimbra looks to be a good alternative to Microsoft Exchange and it has big advantage - it is provided as open source for free in its basic edition.
Zimbra is a multiplatform application, being developed and supported on Microsoft Windows, Mac OS X and Linux to name the most common operating systems.
Zimbra provides integrated email, calendars and documents. But it is not all - it also provides VoIP, but I had no chance to try it.
When you need additional functionality from the collaboration suite, you have pretty well documented API and you can develop your own small extensions of the suite - they are called zimlets.
Zimbra Collaboration Suite has its value and you can definitely try it.

As an Apple and Zimbra user, one irritating thing that has nothing to do with Zimbra per se, is how the Mac mismanaged invites, especially coming from Exchange, which we sadly have to deal with in our x-platform environment.
Just found this, which may help. http://blog.01.com/?p=323
In any event, Zimbra’s great, been using it since 4.0, and it’s improvements are swift (although never swift enough). Looking forward to Z5, and iZimbra!