The nRF24L01+ ICs are 2.4GHz transceivers that only need a few external parts. They’re controlled by a simple SPI protocol, and two extra pins. The IRQ pin signals when data is available and can be used to wake a microconroller with an interrupt. The CE pin is an input activates/deactivates the transceiver.
126 channels are possible, each channel can support one master and 6 slave devices. It is also possible, with a microcontroller, to build a master unit that communicates over all 126 channels.
Nordic modules are available on eBay for a few bucks. We found the reference design they all seem to be based on in the nRF24L01+ evaluation kit archive. nRF24L01pluss_REFERENCE_MODULES.pdf documents the modules, and the reference module gerber files are included if you want to make your own boards.