In the United States, most radio stations are licensed by the Federal Communications Commision (FCC). However, if a transmitter has very low power and is unlikely to interfere with other radio stations, then it may fall under the category of "Part 15 Devices" and it may be legal to operate without a license.

Here are a few of the most common types of low-power, unlicensed transmitting systems:

Carrier-Current AM 

Transmits AM radio signals through a building's AC power wiring.

Leaky Cable FM 

Similar to carrier-current, except that instead of using the building's existing wiring, a special type of "leaky" coaxial cable is run through the building, usually down hallways.

Micropower Stations 

These stations simply use very low power transmitters to broadcast their signals to a small area, such as a college campus.