Identi.ca is a microblogging service. Users post short (140 character) notices which are broadcast to their friends and fans using the Web, RSS, or instant messages.
Identi.ca runs on the Laconica microblogging software which is open source and released under the AGPL. Laconica allows you to set up your own microblogging instance which can be connected to other instances (such as Identi.ca) via the openmicroblogging protocol.
The following are a few of Laconica's features: