The Crazyflie Nano Quadcopter Kits are open source, and designed for flexible development and hacking. It's among the smallest in the world, weighing only 19 grams and measuring 9 cm motor to motor. This thing is tiny.
Due to its small size, it's perfect for developing new software and can easily be flown inside. It can lift a payload of up to 5-10g. This 10 DOF version integrates sensors for altitude and heading measurement as well.
The Crazyflie is open source, so schematics and firmware/host source code is available at the Bitcraze website as well as a Wiki with design information and details on how to set up the development environment and get started. It's designed with development in mind and has an expansion header that enables developers to expand the platform.
Please note: this kit requires soldering for assembly and is not recommended for beginners. In order to get flying, a few items that aren't included in the kit are needed. Please see the "specifications" tab above for more.