This is a, well, tiny FM transmitter that can have various applications both *probably* legal (pirate radio, your own low-power radio station...) as well as the undoubtedly illegal (FM jammer, spy bug...). The choice is yours!
It can run on a voltage between 3 and 13 volts, draws 2-3 milliamps and has a theoretical output power of ~3 milliamps (although you could easily add an RF amplifier).
I attempted to use a CR2032 with the circuit and, to be honest, i wasnt too sure if it would work. I suspected that something weird would happen when feeding only 3 volts to the 3.3 volt regulator or the vco being too sensitive, etc. However, none of those scenarios came to be true and it worked wonderfully. Although i suspect the output power has gone down, there were no issues and the frequency stayed rather stable.