lucidscience.com has build a 2 transistor Spy Transmitter that will send the sounds picked up in a room to any FM radio tuned to the same frequency as the transmitter. Check construction details on the link below. [via]
This project is focused towards those who have not yet attempted to build any kind of RF project, and it is laid out in such a way as to make it easy to explore the basic principles of RF circuitry and ensure a successful final product.
Tiny 2 transistor Spy FM Transmitter - [Link]