RDForum is a discussion forum centered around radar detectors, laser jammers, police speed enforcement tactics, and related information. RDF community was the first to beta-test the application and provide very valuable feedback.
ADS-B Exchange is a world's largest co-op of unfiltered flight data and an excellent service collecting and sharing data from ADS-B aircraft transponders across the globe. ADSBx helps us getting real-time aircraft positions.
The OpenSky Network is a community-based receiver network which continuously collects air traffic surveillance data. They also have a releasable aircraft database which significantly helps us with filtering out safe aircraft.
POI factory provides awesome databases of speed cameras for USA and Canada. Highway Radar uses it to warn drivers about approaching traffic cameras.
Highway Radar is a non-commercial application, so you don't need to pay anything for using it. However, if you wish to support the development of this application you're welcome :)