Twitter is a free service and only requires creation of a login account. Users can then post and read updates via their Twitter homepage or they can add cell phone numbers, IM accounts and email accounts for receiving and sending Twitter updates. More information on how to add devices and use Twitter can be found on the Twitter FAQ page.

Using Twitter as a true "instructional technology" may be a stretch. However, understanding the uses of Twitter and other tools like it becomes important as a way to understand more about how students connect to each other and deal with distributing information. Several other social networks, like Facebook, also have quick-updating, Twitter-like status applications for creating short messages. The possibilities become greater as a network expands to more than just a few people. For example, an entire class on Twitter would be more likely to be contacted if class gets canceled than they would if the faculty member only sent out an email since a Twitter update would go to cell phones and other mobile devices as well.
Clive Thompson proposes the importance of Twitter as a tool for metacognition in the Wired article: Clive Thompson on How Twitter Creates a Social Sixth Sense.
Educause article: "7 Things You Should Know About...Twitter"
The "Newbie's Guide to Twitter" also has some useful links and information
See real-time updates appear on a world map at http://twittervision.com/
See local/regional activity using twittermap
Twitterholic tracks and lists Twitter users that update the most and/or have the most followers
Mashable has a short article on Twitter applications for Firefox