ReTweetability Index: http://www.retweetability.com — A tool created by Dan Zarrella to measure how retweetable any twitterer is. Still does not recognize @johnkremer on April 14th. Not sure why. Odd. Alexa rank: 5,570,588 on March 6th; 238,904 on April 14th.
Tweeple Rank: http://www.tweeplerank.com — Ranks the top tweeple for #FollowFriday, #MrTweet, and #TweepleTuesday. Alexa rank: 376,974 on April 10th.
Tweet Stats: http://www.tweetstats.com — Allows you to track your Twitter account for tweet frequency, daily average tweet density, aggregate hourly tweets, who you are @replying to most, what interface you use to access Twitter, and more. Can also view the stats of other Twitter users. Alexa rank: 58,141 in early February; 42,707 on April 14th.
Twinfluence: http://www.twinfluence.com — Measures a Twitter users reach, velocity, and social capital. Tells you how many second-order followerers you have. Alexa rank: 69,815 on March 9th; 59,900 on April 14th.
Twitalyzer: http://www.twitalyzer.com — Ranks any user for influence, signal, generosity, velocity, and clout. See the site for definitions of those terms. Alexa rank: 601,332 on March 18th; 67,738 on April 14th.
Twitter Break: http://www.twitterbreak.com — Features a tweeter of the day. Not sure how you get featured. Also features little games and trivia to break up your twittering. Alexa rank: 241,491 in early February; 201,162 on April 14th.
Twitter Counter: http://www.twittercounter.com — Provides stats on anyone you want to follow. Includes the Top 100 twitterers. You can pay to be the featured twitterer. Alexa rank: 163,316 in early February; 7,973 on April 14th.