This weekend if found Group-Office which can be downloaded here
as a free opensource version. Group-Office includes more features than Roundcube: calendar, file uploads, RSS feeds, ToDo lists...
With plugins the funcionallity can be expanded very easy.
The installation is a bit tricky but the installer always shows when there is a problem and suggest a possible solution. The nice thing about Group-Office is that you can share mail or calendar with other users which have an account on Group-Office. It feels more or less like a real office application.