Here is a neat project for designing little FM transmitters to spy on your friends or the Boss at the office. These can be built very inexpensively and tuned in on any FM radio. The trasmitters operate on a frequency range of 88 to 108 MHz. They are similar to he ones they sell at Radio shack. I have used several commercial ones, and the range is pretty good depending on the length of the attenea.
Check it out here.