Bill Ives posts 25 Creative Twitter Applications. He embeds this link.
Examples go from really useful -- but dangerous if you text while driving:
CommuterFeed aggregates tweets about traffic from people stuck in jams. By following your city’s feed, you can get updates about ongoing traffic tangles. New Jersey Transit riders have created a similar service.
To just plain goofy:
When Tyler Aaron Menscher (@kickbee) was born in January, he already had hundreds of followers. That’s because his dad, Corey Menscher, created a device that recorded and tweeted his son’s kicks from the womb as they happened.