This service uses a Free microblogging tool called Laconica. Laconica is available under the GNU Affero General Public License Version 3.0, a Free Software license for network services.
You can get a copy of the software from the Laconica main site. The version of the software that runs on this site is unmodified from that version. The site also depends on certain libraries and other software; you can get those at the Laconica site, too.