We just love this site. Beyond being mesmerizing, it serves as a cool messaging tool at events. Holding an event where monitors are available? Go to Visible Tweets, enter in your twitter name, click full screen and watch people become hooked on reading your organization's tweets. The service also pulls in all mentions of your twitter account adding an even more powerful, and interesting, effect.
Lots of fun and an effective way of getting people to follow you on twitter. http://visibletweets.com