Twitter Grader claims to measure the “relative power of a Twitter user.”
If you're trying to find people by topic, Twellow.com is a great place to start. (Results are sorted by whoever has the most followers first.) And TwinInfluence.com claims to measure the combined influence of twitterers and their followers, as does Twitterank.com. But if you're looking for more of a “Who's who,” TwitterGrader.com seems to be a good place to start.
One of the nice things on TwitterGrader.com is their suggestions of who to follow. Just enter your own twitter ID in to get graded, and a list of suggestions is automatically generated. Refresh the page for more suggestions.