Twist – See trends in Twitter.
Twitter Search – See what’s happening — right now.
TwitterVision – A real-time geographic visualization of posts to Twitter.
Twitterholic – Top Twitter user rankings & stats.
TweetWheel – Find out which of your Twitter friends know each other!
TwittEarth – Live Twitts all over the world in 3D!
TweetStats – Graphin’ your stats.
TweetVolume – See how often words or phrases appear on Twitter.

Have fun!