In their own words

FullContact is the API that keeps contact information current. Developers that have applications with contact information can utilize the API to clean, enhance, and keep that information fresh and up-to-date. We do this by aggregating billions of contact records, all with numerous attributes, including quality, freshness and frequency. Our patent-pending algorithms process all of this data to automatically produce clean, accurate full contact records, complete with rich social profiles.

Visit FullContact.com, sign up for a free API key, and put the power of FullContact information into your apps today!