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.
I am a CELTA Certified ESOL teacher living in Germany. Before moving to Germany I worked at Discovery Education as a DEN Manager and as a Staff Development Coordinator and teacher in Pennsylvania.