openatrium - a group collaboration application
Openatrium.org is a drupal (in core) group collaboration application. You can create groups and switch on features for document versioning, calendaring, shoutbox and you can send email notifications to group members. It also has a dashboard for each group that lists all recent activities or posts in the group space. Nice thing is that it has a neat interface, with easy to understand bubble notes to users and on top of that spaces can be color coded (even your personal space ;) ).