OpenMicroBlogging
The first implementation of the OpenMicroBlogging protocol is the Laconica software, which changed name to StatusNet in August 2009. Identi.ca is the first service to support OpenMicroBlogging, and other sizeable services including Leo Laporte's Twit Army at https://web.archive.org/web/20080826063959/http://army.twit.tv/ were amongst those powered by the open source StatusNet. Since March 2009 one can search users' accounts in Twit Army from within Identi.ca. You could also subscribe to accounts at Twit Army from your Identi.ca account. The second implementation of the OpenMicroBlogging protocol is the OpenMicroBlogger software.
History
It lets the user of one service subscribe to notices by a user of another service.