As I've said before a new twitter application is born about every 30 seconds in the world, but a listener “Brian” pointed out three that he finds useful.
This service shows you how many tweets you've posted, and when you posted them (which day, etc). You can find it at www.tweetsats.com
You can find this one at www.twittercounter.com
Twitter counter is pretty darn cool. Not only does it show your growth over some set periods of time it’ll also show you your average growth per day.
It’s been fascinating to watch how the growth changes depending on a number of different factors such as if I auto-follow everyone who follows me.
Last but not least is Twitter Grader at http://twitter.grader.com/
This one is useful when you want to connect with some of the larger twitter users in a local area (like at conference, etc). It's pretty much a ego meter.