OpenGroupware (http://www.opengroupware.org is a OpenSource (GPL) Groupware solution. Most of the other solutions that are available and are OpenSource are PHP based solutions. These do not provide the possibility to use handhelde devices, or external programs to use/manipulate the data (Calendar,Contacts, etc)...
Functionality wise, OGo seems to be a better an more mature solution. In this document I will attempt to install the server, and use mail and calendaring functions via external applications (Mozilla, Evolution, Outlook ... etc), as well as try to syncronize with PDA's