Prior to releasing my own twitter app, I thought I'd just check and see what other existing twitter statistic apps there are. I found TweetStat, which had very interesting stats on my own twitter usage and also pointed me at another site to make a Wordle - pictured here.
I also found Mr. Tweet which seems to require me to follow them before I can find out more info about stats of friends and followers. I'm less enthusiastic about that, but there model seems to be to push out stuff over the twitter wire ... will blog when I get something interesting from them. Seems like this might be a slow service. TweetStat was remarkably fast given that I didn't have to sign in.
I found both TweetStat and Mr. Tweet in this blog post.