Retweet Rank allows anonymous or authenticated lookup (via oAuth) on the retweetability of a user. The formula seems to be easy. It is the ratio of the number of times a person is retweeted. I am not sure how the percentage helps you. I imagine the more your followers followers retweet you is even better, no?
The do run an ad down the right side when you use the service. Nothing major, but at least they are trying to monetize.