I have been working for some time on a quadcopter project based on a Arduino MEGA. There is nothing fancy about it compared to other quad but it might help other getting started:
I wrote the code mainly from scratch. It is on Github: https://github.com/GoussLegend/quadcopter
I wrote also a brief report: https://drive.google.com/file/d/0B2eCSBjoPP37Y2ZKR3dkUWhOWkU/edit?usp=sharing
For now the development is stopped because I moved abroad for some time, but at least the quad flew before I left, so the code should be somehow right
And I'd like to thanks Basel, who helped me for this quad on this forum! http://forum.arduino.cc/index.php?topic=184503.0
If you have any questions, feel free!