Laconica [1] is a FOSS microblogging platform. 

Laconica (pronounced "luh-KAWN-ih-kuh") is a Free and Open Source microblogging platform. It helps people in a community, company or group to exchange short (140 character) messages over the Web. Users can choose which people to "follow" and receive only their friends' or colleagues' status messages. It provides a similar service to sites like Twitter, Jaiku, and Plurk.

Want to try Laconica without installing anything?> Try identi.ca [2]