Who Should I Follow 
Posted on May 14, 2008
Twitter Friend Recommendations
http://www.whoshouldifollow.com/
Find New Twitter Friends.
Who Should I Follow 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.