ardupilot.org

ArduPilot is a trusted, versatile, and open source autopilot system supporting many vehicle types: multi-copters, traditional helicopters, fixed wing aircraft, boats, submarines, rovers and more. The source code is developed by a large community of professionals and enthusiasts. New developers are always welcome! The best way to start is by joining the Developer Team Forum, which is open to all and chock-full of daily development goodness.