The nRF24L01+ module (sources (archive.org)) is a really cheap & easy way to achieve wireless communications. With the internal antenna, the reach is about 100m (open area), with an extra antenna it’s apparently 1100m+.
One popular Arduino library for it is the Mirf library (archive.org).
Connections Required pins with fix locations / can’t be moved:
Arduino Uno / Nano (archive.org):
MISO -> 12 MOSI -> 11 SCK -> 13 Seeeduino Mega 1280 (archive.