eGroupWare is a groupware server. It comes with a native web-interface which allowes to access your data from any platform all over the planet. Moreover you also have the choice to access the eGroupWare server with your favorite groupware client (Kontact, Evolution, Outlook) and also with your mobile or PDA via SyncML.
See also:
http://www.simple-groupware.de/cms/
http://en.wikipedia.org/wiki/List_of_collaborative_software