LowFERs are Low-Frequency Experimental Radio and they are the strictly experimental communications in the low frequency band. Many of these broadcasters are hams, but no license is required to operate here under 1 watt of power. LowFERs experiment with longwave signals below 500kHz. The part 15 rules limit the total length of their transmission lines antenna and ground lead to 15 meters. The have a club of course http://www.lwca.org/
MedFERs are Medium-Frequency Experimental Radio (as you might have guessed) and operate between 510 and 1710 kHz. This is called the mediumwave band. Broadcasts can only operate at a power of up to 0.1 watts. There is good list of these broadcasters here.
HiFERs operate in an even higher frequency band. It's 14kHz-wide band and centered at 13.56MHz. But here power is restricted to a very weak 4.8 milliwatts.
Also related are BeFERs and 49ers. BeFERs are licensed in canada to a narrow band between the Low and Medium wave bands at about 6,700 kHz. 49ers operate at about 4,900 MHz a designation legal in both Canada and the U.S.
Popular operations in these ranges are more code, some digital broadcasts and beacons. Some even send out their own QSL cards. As low as the limits are on power, some operators have managed to send a signal over 1000 miles. Any type of modulation is permitted within the band. The most important rule is that A Part 15 operator must not cause interference to any licensed service. But in turn, they must tolerate any interference from other licensed services. If that wasn't clear enough, the man made interference is endemic to these bands. We call it QRM. There are some clips of beacons in the noise here.
Could you be illegally using a device that does not comply with part 15? Yes you could. You may never be caught, but it's not entirely uncommon. If you remember last year it came out that almost all XM and Sirius FM translators (the thingee that makes it come out your car radio) operated illegally above their allowable power. The idea that other devices have the same problem is credible.