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.

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.
More twitter stuff.
Anyone using?