Radio
National Radio Data - http://www.nationalradiodata.com/
An on-line subscription-based provider of nationwide scanner frequencies, trunked frequencies, ham radio, and mass media for DXing. National Radio Data also provides worldwide airport frequencies, civil aviation registry information, public service information, and more.
Homing In - http://www.homingin.com/
Find what equipment is used, plus dozens of links and local contacts to help find RDF activities in your area.