DoesFollow

Keep tabs on who follows whom on Twitter

There's no need to troll through someone's following list to see if they are followed. DoesFollow tells you in no time who follows whom, whether they are celebrities, political figures, or your average Joe/Jane.

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Discussion
Simple and effective. For a slightly more advanced tool, check out followerwonk (which is owned by moz.com)
I love how Refresh does this in their app. Giving me the skinny of who the person I am meeting with follows (who is relevant to me).
I made this.