The OPEN-XCHANGE Collaboration and Integration Server Environment allows you to store appointments, contacts, tasks, emails, bookmarks, documents and many more elements and share them with other users. This environment can be accessed via any modern web browser and multiple fat clients like KDE Kontact, Apples iCAL, Konqueror, Mozilla Calendar and many more.
How I wish for something like this .. something to offer a real alternative to Microsoft’s Exchange/Outlook “solution”; read more about it here.