Last year, there were several announcements for open source tools designed to become Microsoft Exchange alternatives. Novell’s Hula Project and Zimbra, are the two most high profile efforts for a viable Exchange alternative.
While developing an exchange replacement is a valiant pursuit, I have not seen a project that can function as a calendar alone. Today, I installed Zimbra, since it seems to be the most mature product on the market.
Zimbra is really slick, its a combination of a bunch of open source tools in to one easy to manage solution. If I were setting up e-mail and calendaring for new company, Zimbra would be high on my list. The problem with Zimbra is that it will not function as a standalone calendar.
A lot of organizations do not want to replace their e-mail solution, they are simply looking for a calendaring solution that is interoperable with outlook and the popular open source calendaring clients.