Title 47 CFR Part 15 is what it is under. That covers everything such as your WiFi, cordless phones, and hobby transmitters. For AM, you are allowed a max of 100 mW with an antenna no larger than 3 meters. FM and SW are also allowed, but their requirements are a lot more stringent (e.g. those iPod-FM transmitters and those real estate "Talking House" transmitters).

A lot of hobbyists have their own stations under these rules. I have an old P1 Win 2K computer feeding an AM and FM transmitter playing OTR shows and music for my enjoyment. When conditions on AM are right I've gotten around a mile coverage with my car radio. (Car radios are much more sensitive than those you use at home.) I got lucky and when a neighbor moved out of her house and let it get repossessed, she also quit the real estate business and left behind 5 of the Talking House transmitters. I run one of those and one of the C. Crane FM transmitters (although mine is the older, first model).

There's no limit on Part 15 stations being for profit, and some are actually run as such. Others are Community radio or hobby stations like mine. The only thing is you are not supposed to use callsigns, since the station is unlicensed, and you can't interfere with a licensed broadcaster. Even though I have my Amateur Radio license, it's still fun to think I have an AM and FM station going, even if it is "flea-power".

Michael
...jeez, do I have enough links in this or what?