OpenGroupWare is a groupware server that uses open standards for communication (LDAP, WebDAV, XML-RPC.) This means that its perfect for using with mulitple clients on multiple platforms. Currently working Clients are Mozilla address, Outlook, Evolution, and Kontact. It has a built-in web interface for use also. It looks like the Exchange replacement of the future may very well be Kontact and OpenGroupWare.
Kolab is the KDE groupware server designed for kontact. Kontact can be used with OpenGroupware, Kolab, Exchange, support in development for eGroupWare, phpGroupWare.