An ultra low-power transceiver for radio communication in the 2.4GHz ISM band (Industrial, Scientific, and Medical). Commonly used by tinkerers because of its low cost and its easy to use SPI interface.
These radio modules are:
- Cheap (1-2£)
- Have relatively long range (some versions have ~1km range)
- Ultra low power (no problem powering these modules for really long periods of time from batteries or even coin cells)
- Easy to use SPI interface if one does not wish to use a library
If you are considering or using nRF24L01(+) modules then the most popular library is this. This library is also available for download through the IDE's built-in library manager so that if you have installed it, it won't be hard to keep it up to date. If this barebones library is too complicated then there is also RF24Mesh and RF24Network libraries, each making the communication even simpler.