Twitter directory is similar to a phone directory. Here you can see who are top users find new friends and find people who tweet about your areas of interest. You can get yourself listed in twitter directories too, this may even get you more followers.
Some of the most popular twitter directories are

Wefollow: this is the biggest and the most comprehensive list of twitter users. Here you can find people of any and every category ranked according to the number of followers they have. Wefollow will not auto include any user so if you want to see yourself there you got to add yourself . adding yourself is very easy you just have to mention 3 hashtags which describe you
Twitter Directory and Search, Find Twitter Followers : WeFollow

Twellow: Ha s a huge number of users listed in. This site categorizes profiles based on information provided in bio. (twellow is combination of twitter and yellow meaning twitter yellow pages!0
http://twellow.com/

Just tweet it: Not too many profiles listed here but there are people in neqrly every possible category
Just Tweet It » THE Directory for Twitter Users!