Open source drone technologies
I can think of a couple of open source UAV projects:


Paparazzi is a complete system of open source hardware and software for Unmanned Aircraft Systems, including autopilots and a ground control and planning station. All of the hardware plans and software source code you need to make a drone from scratch are available on github. But not everyone has the equipment to make a printed circuit board at home.
Open pilot is an autopilot for UAVs. They also sell hardware if I remember properly, which might make it a bit easier if you're just starting. 


If you just want a simple introduction to flying drones, DIY drones has a quick guide to the basics.