Scalable OGo (SOGo) is a free groupware server focused on scalability instead of depth in functionality. The web interface uses human readable URLs and can be accessed according to REST web service ideas. It requires no server side session.
The server stores data in the iCalendar/vCard formats and therefore has no data model inconsistencies with native clients - it easily connects using GroupDAV connectors.
Scalable OpenGroupware.org is being developed since August 2004.