Twitter

Twitter is a free social networking and micro-blogging service that allows users to send updates 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. Users can receive updates via the Twitter website, instant messaging, SMS, RSS, email or through an application such as Twitterrific or Facebook. For SMS, four gateway numbers are currently available: short codes for the United States, Canada, and India, as well as a United Kingdom number for international use. Several third parties offer posting and receiving updates via email.