Is there a numerical cap on how many friends we can have? Many cite the so-called Dunbar's number, 150, as a ceiling on personal contacts, but social-networking sites may enable users to burst past that number for friends."
WhoShouldiFollow looks at who your Twitter friends follow to suggest new people. By adjusting the sliders on the results page, you can give greater preference in the ranking to less popular Twitter users, or those who are near you.
The algorithm is an item-to-item collaborative filter, somewhat similar to how Amazon makes recommendations.