TweetWheel - find out which of your Twitter friends know each other
Twitter Blacklist - test a user name to see if they are a fake or Twitter spammer
I previously posted on Twitterlocal, where you can get a stream of tweets from people around a location.
Now they have a new leaderboard feature on the sidebar which lists the top tweeters in that stream…they also have a top cities leaderboard list.
Check out top tweeters and a stream of tweets from people in Perth, Australia.