Twitter has added an alphabetical user directory on their home page for visitors who are not already logged into the social network.
Having only discovered this today myself, I found an article by Matt McGee over at Marketing Land where he claims the directory was launched "a few weeks ago ~ but hasn't made a formal announcement about it".
Twitter's aims are perhaps to get users' profiles ranking in the search engines to attract traffic to their service that way. From my own personal experience of coming across profiles via Google, it takes me seconds to decide whether I want to follow that user or not before moving on. Twitter is struggling to give people a reason to remain on a user's profile for a sustained period of time. However, I do love Twitter and spend lots of time on it in case they were interested to know ;)