Groupware
E-mail, calendars, address books, task lists... are you worried about entrusting all your data to a proprietary server which incurs considerable license costs and forces you on a regular update path?
Then you need a free Groupware solution: The Kolab server is a good alternative to Microsoft Exchange (R).
Existing Outlook clients can be re-used via the inexpensive, commercial KONSEC Konnektor. And with the KDE groupware client Kontact running under Linux you can access the Kolab server without any licensing costs. It is also possible to access data over the web - not only for the users, but also for the administrators.
A free, simple and inexpensive groupware solution. It is web based and so accessible to by all platforms. A basic binding to Outlook is also available.
An extensive and flexible free groupware suite. Complete and reliable bindings for Outlook, Linux and web clients.