There are two very popular twitter directories, which you can use to find twitter users you can follow based on interests.
1. Twellow is a twitter yellow pages.
It is known as the twitter directory and has hundreds of categories and it is good toll when you want to find some users tweeting about obscure niches and follow them.
You can create an account in Twellow, list up to 10 categories you are going to be listed in, and create a bio.
2. Wefollow is another twitter directory arranged according various categories.
You can tag yourself by sending a tweet to @wefollow, or registering for free on the website by pointing out the categories you are interested in.