The Sharetronix suite of enterprise collaboration tools is in beta and free to everyone. Unique to its suite are usage statistics and the ability to have content indexed by Google (if you want).
Features: Status updates, real-time dashboard, groups, ability to share multimedia, content tagging, search center, real-time RSS aggregation, Facebook and Twitter integrations, mobile capabilities, reshare function (similar to Twitter's "retweet" button), open API and more.
