Visit this link:
http://www.emilychang.com/go/ehub/app/getting-things-done-with-twitter/
Don’t be put off by the title if you’re not a GTD-type person. This list is a cornucopia of services built on top of Twitter, many that I’ve never heard of until now.
After three years on the web, Twitter has become more than just another micro-blogging client. A whole industry of web applications have sprung up to extend and capitalize on Twitter’s capabilities, from the ridiculously silly, to the gobsmackingly useful. If Twitter is not already a part of your daily life, these applications can give you a whole new perspective on the idea of Getting Things Done. There’s a lot you didn’t know you could do with 140 character spurts.
Update: As someone just pointed out to me… if you thought that was a decent list, check out the Twitter Fan Wiki. Whoah.