1. Twitter People Search – Twitter's People Search may not be the best way to find people on Twitter but it certainly is the easiest. If you're just getting the hang of Twitter or are a new user, this option will be available directly on Twitter.com. The Twitter People search searches the real names Twitter users enter in their bio fields. It's not mandatory to place your real name so that can be an issue if you may be searching for a certain someone; especially ones whom have the same name. This I'd say is a good place to start.
2. Tweepz – "Search, Find and Discover Interesting People on Twitter". It's as easy as that. Tweepz.com lets you limit searches to specific areas of Twitter’s user info (ex: name, bio, and location), filter results by follower/following numbers, and location. Overall, Tweepz acts like a Twitter search engine.
3. WeFollow – WeFollow is more organized and fun to use. It offers a wide Twitter user directory that organizes people by hashtags, categories, tags and cities. WeFollow is user-generated and anyone can add themselves by tweeting @wefollow with three #hashtags that describe them.
4. TweepSearch – Although it may be a bit similiar to Tweepz, it doesn't get as much results as Tweepz would. TweepSearch allows you to search by name, location, or a specific username to get a list of friends and followers.
5. TwitDir – TwitDir acts as another Twitter search engine. The only feature that TwitDir offers that other engines don't is that it has a few links that list the Top 100 Followed, Top 100 Updaters, Top 100 Favorites and Top 100 Followers.
6. Twellow– Twellow is good if you're looking for Twitter users with the same interests or jobs. It acts like the good ol' fashioned yellow book directory by listing a bunch of categories (Entertainment, Food, Geeks, Science, etc.) Twellow is certainly one of the most advanced of them all. Around 6 million Twitter user profiles are indexed and placed into a huge number of categories. It also has a community called the 'Twitterhood' which you can join.
7. Just Tweet It – Acting somewhat similar to WeFollow, it’s not as well organized or easy to navigate. Best thing about it is that it has a large community to browse around to find people that you would like to follow.