Klout says it measures influence across the social web. To sign up, you enter your twitter ID, so your Klout score is based on your impact there.
Klout collects data about the content you create, how people interact with that content and the size and composition of your network. The Klout Score ranges from 1-100 and represents a person’s overall influence.
Both websites Twitalyzer and Twitter Grader tread a similar path. Great fun I guess but hard to see any authoritative value of any of this data tbh and essentially a waste of time unless you need your ego stroked/shot down in flames!
Thanks to @DannyDee.
Further Reading
With Klout Comes Influence: Measuring And Modifying List Authority on Twitter (Mashable)
