GNU social is a free social networking platform. It helps people in a community, company or group to exchange short status updates, do polls, announce events, or other social activities (and you can add more!). 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, Google+ or Facebook, but is much more awesome.
With a little work, status messages can be sent to mobile phones, instant messenger programs and specially-designed desktop clients that support the Twitter API.