Twitter has launched (without any formal announcement) a new profile directory, which will allow you to browse through Twitter users in alphabetical order. Some accounts do seem to be missing, however.
Amit Agarwal at Labnol.org (via Matt McGee) suggests that the directory is aimed at making Twitter profiles more visible to search engines. "People directories, likes the ones now available on Twitter and Facebook, aren’t necessarily useful to human visitors (unless you are trying to research particular names or surnames) but they are like sitemaps that help search engines bots discover every single profile that exists on the social network," he writes. "Twitter’s people directory doesn’t have a search function but you can take the help of Google to find the existence of your name in that massive directory."
To use Google for this, he suggests you simply search: site:twitter.com/i/directory/profiles "your name"
Twitter is displaying a link to the directory in its footer for logged out users, though Agarwal is right in that it hardly seems geared towards users without a search feature (the search box simply searches Twitter via normal Twitter search).
It doesn't offer much but a lot of names without any context. If you're looking for Twitter users of interest, you're still better off with a tool like Twellow.
Twitter's directory is good if you want to see how many other Twitter users have the same name as you:
Disclosure: Twellow is operated by WebProNews parent company iEntry.