We've covered several utilities that have found fun and creative ways to analyze Twitter messages, but TweetPsych takes the cake. This one looks at your past 1,000 Twitter posts and gives you a "psychological" profile, including how much you talk about yourself, work, money, and "negative emotions."
In other words, it's a great way to reinforce the fact that you're probably using Twitter for self-promotion, and/or as a way to kvetch. At least that was its analysis of my tweets.
In an introductory blog post about the tool, creator Dan Zarrella says the it works by cross-referencing the words and phrases you use in your tweets to two different dictionaries that are sorted into various psychological profiles. It then scores you in each category based on the results of other TweetPsych users. This makes it less about psychology and more about your personal lexicon, but the results are still quite fun.
The service works with any Twitter account, meaning you can use it on anyone else you know. As mentioned earlier, it only pulls from the last 1,000 or so tweets you've made, so the results will not be nearly as detailed if you're a new Twitter user.