Visual Impact Muscle Building : Open atrium enables teams to collaborate within a single site.
Six features come bundled in, and Open Atrium can be extended to include additional features (see here for a list of features contributed by the community):
Blog: A classic blog for each group that has commenting, file attachments on both the post and comments, and granular notifications.
Calendar: A calendar that lets you quickly add events, suck in iCal and other calendar feeds, and support single or multi-day events.
Dashboard: Mini widgets give you a snapshot of all the activity happening across your groups. Think iGoogle, in that you can turn on what you want, turn off what you don't, and arrange it all however you'd like.
Notebook: A simple online handbook that lets you collaborate on documents, store and compare revisions, attach files and, once you're done, print out the final copy.
Shoutbox: Kind of like a private twitter, the shoutbox lets you share short messages, links, and information with just the people in your group.
Case Tracker: A full ticketing system that lets you assign to dos and create unlimited projects within each of your groups. The case tracker also lets you classify the to do, give it a priority, and manage its status.
Open Atrium is completely customizable. If you want a feature that's not on the list above, you can add it yourself. Read the How to Build a Feature section to find out how to do it.
The fun really starts when people start sharing the features they build for their own Open Atrium with everyone else. Then everyone doesn't just have access to six features but also a bunch of others that are easy to plugin to the basic Open Atrium package.
Take a look at the Contributed Features page for help on sharing features, as well as a list of features are being developed by the Open Atrium community.
Below are a few videos to help you get started with Open Atrium: