Scambio is a Free alternative to online services such as GoogleApps or MobileMe. It is also much simpler, so that you can easily extend it to fit your needs.
As of today, it supports mail, addressbook, calendar and file sharing. In the future it will also support realtime communications like VoIP.
It uses custom but very simple protocols to synchronize meta-data and file contents. It comes with a small client GUI adapted to mobile devices such as the Nokia N800 serie.
For more explanations and a longer description, see the Documentation.