Twitter

From Wikipedia:

Twitter - Twitter is a free social networking and micro-blogging service that allows users to send updates (otherwise known as tweets) which are text-based posts, ranging up to 140 characters long.
Updates are displayed on the user's profile page and instantly delivered to other users who have signed up to receive them. The sender can restrict delivery to those in his or her circle of friends (delivery to everyone is the default). Users can receive updates via the Twitter website, instant messaging, SMS, RSS, email or through an application such as Twitterrific or Facebook.
Who is using it?
Dell is using Twitter to respond to customer queries, send word of promotions and provide service updates, among other things. Direct2Dell
Many organizations have embraced the technology, including the Los Angeles Fire Department that put it to use in situations such as the October 2007 California wildfires.[5]
Higher education is also using the technology to relay important information to students in a more timely manner. Such is the case with The University of Texas at San Antonio College of Engineering.[6]
Several U.S. presidential campaigns use Twitter as a publicity mechanism, including Barack Obama.[7]
Media outlets such as CNN have also started using Twitter to break news.[8]