Co-op, a no-frills application with a simple user interface similar to Twitter's, is free for everyone.
Cynapse's Cyn.in open-source community edition has a robust collection of enterprise collaboration tools. Three versions of the tool are available; the free community edition includes Active Directory integration, application source code, Web-based appliance management console and more. The other editions sell for approximately $4,500/year to $15,000/year.
Features: Wikis, blogs, file repositories, event calendars, discussion boards, image galleries, collaboration spaces, status logs, people directory, crowd rating and voting and more.