Some open-source electronic projects involve radio communication.
Some amateur radio operators enjoy building custom radio communication systems. Some of them have built systems with features unavailable from any off-the-shelf system, such as software-defined radio.
Other people typically use a pair of off-the-shelf Integrated Circuits#RF ICs/Modules in the unlicensed spectrum as a kind of "virtual wire" between two systems. Such as, for example, Xbee wireless module, etc.
The Open Mobile Gadgets project(s) use(s) the cell-phone-licensed spectrum to build an open-hardware that talks to cell phone towers like a cell phone, but does things that no closed-source cell phone can do.
Open-source RC flying craft (multi-rotor, etc.) typically use FM modulated servo control the aircraft remote control reserved frequency bands (72 MHz ... and others ...) or use frequency-hopping (FHSS) in the unlicensed spectrum.
Open-source RC surface craft (RC cars; RC boats; RC submarines towing a floating antenna; etc.) typically use (FM modulated???) servo control the surface vehicle remote control reserved frequency bands (73 MHz ... and others ...) or use frequency-hopping (FHSS) in the unlicensed spectrum.
(FIXME: mention other "wireless" technologies, such as acoustic, ultrasonic, infrared remote control, free-space optical, etc.)