Coming in at just 19 grams and measuring 9 cm motor to motor is the Crazyflie Nano Quadcopter. This guy is designed for flexible hacking and development and has limitless possibilities.
The kit comes with an on-board 32-bit processor, gyroscope, accelerometer, and damn a compass and altimeter too. You control it via a USB transmitter you plug into your PC and has the potential to be controlled by your cellphone as well. A tiny lithium-polymer battery powers the drone and recharges with a micro-USB and has a drive time of about 7 mins.
With open-source software and plenty of support from the community you could have hours of entertainment with the Crazyflie.