TweetBios - extend your Twitter profile…more info about yourself, links to all your profiles, etc…
TweetEffect - find out which of your Twitter updates made people follow or leave you.
Tweeterate - an alternative interface where you can rate tweets
TweetIE - a plugin for Internet Explorer…these are far and few between
TwittBot - allows multiple people to publish to a single Twitter account, and for a single person to post to multiple Twitter accounts.