Ardupilot is the most advanced, full-featured and reliable open source autopilot software available. It has been developed over 5+ years by a team of diverse professional engineers and computer scientists. It is the only autopilot software capable of controlling almost any vehicle system imaginable, from conventional airplanes, multirotors, and helicopters, to boats and even submarines. And now being expanded to feature support for new emerging vehicle types such as quad-planes and compound helicopters.
Installed in over 1,000,000 vehicles world-wide, and with its advanced data-logging, analysis and simulation tools, Ardupilot is the most tested and proven autopilot software. The open-source code base means that it is rapidly evolving, always at the cutting edge of technology development. With many peripheral suppliers creating interfaces, users benefit from a broad ecosystem of sensors, companion computers and communication systems. Finally, since the source code is open, it can be audited to ensure compliance with security and secrecy requirements.
The software suite is installed in aircraft from many OEM UAV companies, such as 3DR, jDrones, PrecisionHawk, AgEagle and Kespry. It is also used for testing and development by several large institutions and corporations such as NASA, Intel and Insitu/Boeing, as well as countless colleges and universities around the world.
Information about autopilot hardware options is available within the Ardupilot wiki.