This is a very cute hack that does something which I thought was impossible: an implementation of an FM transmitter that has exactly two components: a battery and an ATtiny45 microcontroller. It's brilliantly obtuse and cool:
The basic idea is to trick the internal oscillator of the Atmel to run at 24Mhz. An internal PLL will run at 4x that frequency, which is a 96 Mhz signal (right in the middle of the FM broadcast band). By varying the frequency up and down, you can generate an FM modulated signal. Beyond clever. Very cool.