I designed the first fully 3D printed quadcopter chassis on Thingiverse. I had a lot of experience with crashing RC planes and helicopters, and after buying a 3D printer, my first thought was how great it would be to design an RC quadcopter where parts could simply be reprinted when they broke.
The first design I came up with was pretty overkill. Everything was far stronger than it needed to be, and as a result it could only accept pretty small propellers. The mk2 version was designed with larger props in mind, and turned out much lighter.
For the electronics I used an Arduino Pro Mini, along with accelerometers and a gyroscope from a Nintendo Wii remote. For software I used MultiWii.