ArduPilot
The worlds most advanced open source autopilot
ArduPilot is the most advanced, full-featured and reliable open source autopilot software available. It is the only autopilot software capable of controlling any vehicle system imaginable, from conventional airplanes, multirotors, and helicopters, to rovers and boats and even submarines. Most recently it has been expanded to support new emerging vehicle types including quad-planes, compound helicopters, tilt-rotors and tilt wings VTOLs.
ArduPilot runs natively on a wide variety of hardware platforms from the very popular Pixhawk flight controller (32bit ARM) to the advanced Intel Aero linux flight controller (see full list here: http://ardupilot.org/copter/docs/common-autopilots.html).
The ArduPilot team is lead by an experienced group of passionate and world class developers from all over the world with expertise in Extended Kalman Filters, control theory, embedded programming, Linux and much more.
A welcoming and friendly group of developers that is happy to share their knowledge with you but also with too many interesting projects to complete on their own. An engaged group of partner companies provides the hardware and financial support for the group. They could use your help!