Tweet Scan has recently begun tracking dents on the identi.ca network. If you've not heard of identi.ca, on the face of it it's a Twitter clone. But in actuality it is the main installation of the first major piece of Affero GPL'd software to hit the web: laconi.ca.
If you'd like to search messages on identi.ca, proceed to http://tweetscan.com/identica
At this moment, rss feeds, email alerts, and other parts of Tweet Scan are still only pointed at Twitter but we plan to grow our support for identi.ca as the platform matures.