TweepDiff is a simple tool where you enter two Twitter account names and view the difference between their followers or those they follow. It works very well when you have large groups of people in common and a close number of of either. A large skew of numbers gives a long list.
I compared myself with a couple others and the results came back fairly quick. Three tabs were presented; common and the two accounts you compare.
You could view the avatar images and bio details on each account, which helps you decide if you should follow them yourself. Each list can be viewed in 10, 50 or 100 per page for faster scrolling.
A great, quick tool to see who you are missing or who is missing you.