Google Offers Sneak Peek at Your Available Spectrum article tells that Google.org is publicizing a Spectrum Database that monitors unused spectrum frequencies (a.k.a. white space) across the US. The aim is to allow industry stakeholders and the public to utilize unoccupied frequency ranges better through dynamic spectrum sharing. The database is undergoing a public trial with the FCC to test its validity.
For more information read Google Spectrum Database documentation. After that Browse spectrum on map to get idea how this system works. Anyone can access the Spectrum Database to search for unused spectrum in your area, and it is planned that in the future registered devices will check from it which frequencies they can use and different areas automatically.