WeFollow - A creation from Kevin Rose, the Digg guy, with the use of Twitter Directory tags to segment Twitter users. WeFollow is heavy on the celebrities.
Twellow - The yellow pages of Twitter Directory Listings. A search directory of people by area of expertise, profession or other attribute listed in personal profiles on Twitter.
JustTweetIt - Just Tweet It is a user directory for Twitter organized by genre to allow for users to easily find other Twitter users to connect with.
TweetFind - Live Twitter Directory to Find Twitter Users and Twitter Businesses.