A few tools which make Twitter slightly more useful / user-friendly than its basic incarnation...
Twitterfeed - use Twitter as a sort of public RSS for your blog
Twhirl - desktop client which makes 'tweeting' and following other folk's updates far easier
TweetStats - want to track your usage of Twitter? This is an easy peasy way of doing it!
TweetVolume - allows you to compare key words to compare current popularity on Twitter
Feedtweeter - allows you to post your tweets to Plurk and vice versa, saving that clicking finger!
twittersearch - search for keywords in everyone's tweets
Hashtags - give you the potential to tag your tweets
Mashable's Twitter Toolbox - oodles more Twitter-ific toys!
Sure there have to be lots more useful ones out there so with luck I'll be able to update this with more soon!