Zimbra is a bleeding-edge Web 2.0 project. It is similar to Google Apps, but built with the business and enterprise user in mind. It is also a competitor to Microsoft Exchange, but goes well beyond Exchange’s capabilities.
While there is an open source edition of Zimbra, it was not released under the GNU GPL, but instead the Yahoo Public License, which is more limited. Further, Microsoft does not have a very good track record on open source software, and there is no guarantee that Zimbra will stay open, even in its current, limited form, if Microsoft purchases Yahoo.
I was hoping to use Zimbra myself when I finally get around to setting up a server, so I am hoping the project survives somehow.