PX4 is part of the Dronecode Project, a shared and collaborative open source project to deliver a complete end-to-end platform for unmanned aerial vehicles (UAVs).
The platform includes: PX4, MAVLink, and QGroundControl. The code is made available under permissive IP-friendly licences that allow wide reuse without legal risk to companies or academia.
A number of other projects work with the platform including: DroneKit, ROS, FastRTPS, AirSim, Gazebo, and RViz to name a few.