Identi.ca is a micro-blogging service based on the Free Software StatusNet tool. If you register for an account, you can post small (140 chars or less) text notices about yourself, where you are, what you’re doing, or practically anything you want. You can also subscribe to the notices of your friends, or other people you’re interested in, and follow them on the Web or in an RSS feed.
While similar to Twitter in both concept and operation, Identi.ca provides many features not currently implemented by Twitter, including XMPP support and personal tag clouds. In addition, Identi.ca allows free export and exchange of personal and “friend” data based on the FOAF standard; therefore, notices can be fed into a Twitter account or other service, and also ported in to a private system similar to Yammer.[source]