Followerwonk: Search by Twitter bio, name, URL, location, more
Followerwonk says of itself: "Who are you looking for? Whether it's new talent, customers, or just friends, we help find whom you're after.Login with Twitter and we'll overlay your follow status. Want precise tracking of new/lost followers?" It's a pretty good analysis tool. I like the option of being able to search through biographies in particular, which gives searchers a variety of ways of ranking and rating people. There's a really fun 'compare users' option, analyse and track followers and so on. It's packed full of helpful information, and you can get a lot of data without having to register to use it.