1. TweetFind (http://www.tweetfind.com) is a directory of users who sign-up to be listed too. It requires an extra step over the first two when you are looking at user on the directory you must click their Twitter link in order to follow them. Categories to search in this directory include Blogs, Books, Business, Celebrities, Education, Employment, Entertainment, Entrepreneurs, Environment, Financial, Food, Foundations, Government, Health, International, Marketing, Music, News/Media, Parents, Pets, Professions, Real Estate, Shopping, Social Media, Sports, Technology, Transportation, and Travel.
4. My Twitter Directory (http://www.mytwitterdirectory.com) is similar to Twellow, but Twiiter accounts must sign-up to be seen here as it is not automatically updated. However this may help you because you know people have voluntarily joined this directory.