Interfaces with KSP with the krpc mod. The new python autopilot can be found here. Uses daisy-chained PID controllers to control attitude and speed.
The python autopilot is meant for airplane control: it prioritizes pitch and roll instead of pitch and yaw to change vessel heading.
Features most of the autopilot control options found on a typical airliner A/P panel. Tunes the PIDs on the fly using current dynamic pressure.