Twitter has grown so fast and is so large that in order to find people to follow, you really need some help. My favorite directory for this is Twellow. Twellow (a service of WebProNews) allows you to search by occupation or niche and has set up a series of categories that cover just about everything. If you do not see the category you want, ask them to add it.
You can either browse these categories or look through the entire list. The list is overwhelming, so I suggest browsing first. When you click on a category, Twellow tells you their name, their Twitter name, the number of followers, when they joined Twitter, and their bio. You can also see some of their tweets.
To login to Twellow, and claim your profile, you need to use the ID and password you use for Twitter. You may already be listed in the directory, but you can edit the profile and add categories. Once logged in, you can see your friends and followers as well. There is also a fun tab called “TwellowHood” that lets you search by geography.
Twibs is a business directory for Twitter that allows you to search by the name of the business. You can sort by the number of followers, or alphabetically. The businesses on the home page are there by popular vote.
Each entry gives you the Twitter name of the company, the number of followers, and the location, as well as their bio. If the company has added their business to the directory, they can pick the labels or hashtags that they want to be known for. Adding your business to Twibs is free, and you do not have to be a mega-conglomerate (like Starbucks).
If you are logged into Twitter, you can tweet the company directly (look for the button on the right), and vote for them to be on the home page. Once you have voted for them, you can then ask your followers to vote as well.
I find Twibs to be very useful when I know the name of the company.
Alltop, while technically not a user directory for Twitter, does have a Twitterati section. Here you can see who Alltop thinks are some of the more famous and/or more vocal Twitter peeps.