A laptop, RC plane, and some GPS sensors are all you need to get stared.

Paparazzi is a free and open-source hardware and software project intended to create an exceptionally powerful and versatile autopilot system by allowing and encouraging input from the community. The project includes not only the airborne hardware and software, from voltage regulators and GPS receivers to Kalman filtering code, but also a powerful and ever-expanding array of ground hardware and software including modems, antennas, and a highly evolved user-friendly ground control software interface.