The ArduPilot project is a family of open source autopilots based on the Arduino open source computing platform. It consists of a choice of ArduPilot Mega (APM) electronics and a range of free software versions for different vehicles

You can buy APM 2.5 for $179 here. Everything else you need for a complete autopilot (flight software and desktop utility software) is free!

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Features include:

• Free open source firmware supports planes, multicopters (quads, hex, oct, etc), helicopters and ground rovers!
• Simple setup process and firmware loading via a point-and-click utility. No programming required! (But if you do want to fiddle with the code, you can with the easiest embedded programming toolkit available: Arduino)
• Full mission scripting with point-and-click desktop utilities
• Can support hundreds of 3D waypoints
• Two-way telemetry and in-flight command using the powerful MAVLink protocol
• Choice of free Ground Stations, including the state-of-the-art Mission Planner which includes mission planning, in-air parameter setting, on-board video display, voice synthesis, and full datalogging with replay.
• Autonomous takeoff, landing and special action commands such as video and camera controls
• Supports full "hardware-in-the-loop" simulation with Xplane and Flight Gear

Hardware includes the following:

• 3-axis gyros
• 3-axis accelerometers
• 3-axis magnetometer
• Barometric pressure sensor for altitude
• 5 or 10Hz GPS module
• 4 Mb of onboard datalogging memory. Missions are automatically datalogged and can be exported to KML
• Built-in hardware failsafe processor, can return-to-launch on radio loss.
• (Optional) Airspeed sensor
• (Optional) Current sensor
• (Optional) 2-way telemetry