I just stumbled on this article on unclutter.com about various productivity tools that are available from various Twitter-Bots.  Below is just a few of them, but you really need to get on over to the Twitter Fan Wiki where there are hundreds all sorted out by platform and sub-application. 

It seems like each day I get more and more addicted to twitter.  It’s amazing how such a simple little application can be so useful!

• TwitterNotes- Let’s you take public (and private) fully taggable notes via twitter.
• Timer - Is a down and dirty egg-timer like application.
• Remember The Milk - Is a more sophisticated timer service should you need greater flexibility.