Some of the autopilot frameworks are based on the popular Arduino platform, which makes it very simple for arduino fans to modify and contribute to the code if needed. Here are the a few of those:
1. Ardupilot: They have excellent hardware and software that produces very stable RC aircrafts.
2. Aeroquad: Aeroquad has a large developer community contributing to the development of this project which is making the platform better everyday.
3: KKmulticopter: They developed a very easy to build solution for a very fun to fly quad/tri copter.
4. MultiWii: Another arduino based platform using the popular and easy to find Wii Motion Plus and Nunchuck sensors. They are currently expanding their hardware support for wide variety of sensors.