NRF24L01 2.4GHz Transiever

The nrf24l01+ is often the best solution for long range communication between Arduino-based devices. They are extraordinarily cheap, do not require a wireless network, and any number of them can talk to each other at once. They have a range of around 1km and should always be under that to ensure good use. They can send more complex signals, but there is no API for using them, so anything more than just sending numbers between them will take more time. If these limitations fit your project, however, the nrf24l01+ is a great option.
For more information about this component, here is a datasheet

Set Up

To connect the nrf24l01+ to an Arduino, there are very specific pins that are required. This is due to the serial communications that are required. Fortunately, most of the serial is abstracted away during the code, but the following pins must be hooked up to the same pins on your Arduino; MOSI, SCK and MISO. Vcc and GND are connected to 3.3v and ground, CE and CSN can be attached to any pin and IRQ is not used when communicating with Arduinos.