Klout is a service that provides all sorts of fascinating metrics about you—and the people you interact with—on Twitter. It measures things like the number of users who are actively engaged by your tweets, the “type” of Twitter user you are, and your influence.
It’s neat, particularly if you’re a blogger. You can sort of tell the ratio of “real” followers versus spammers and people who are pretty much ignoring your tweets, based on the “true reach” score. And I find it interesting to see how it maps interactions and figures out who you influence, and who influences you.