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From the early days of Twitter, Twitterholic (later renamed to Twitaholic because of Twitter's trademark on the term "twitter") was the place to go if you wanted to see how you rank against other Twitter users when it comes to number of followers and other factors.
Amsterdam-based Twitter Counter came later, but obviously had a better financial plan and backing. The company claims it's profitable with more than $500,000 in revenue in 2010, and it managed to acquire the older service, which now makes it the definitive authority on Twitter user stats.