Last year I decided to build a quadcopter as a personal project. I spent a lot of time over the summer of 2015 designing a circuit board for it pictured below. It is a 4 layer PCB with more than 300 components. I have planned out most of the mechanical design, but I haven't started assembling it yet.

Quadcopter PCB

I ended up spending a lot more time on it than I had originally planned and added arguably unnecessary features, but I'm really happy with the result.