Arduino 433Mhz RF Wireless Data Transfer
runtimeprojects.com has a tutorial on how to use 433Mhz modules to communicate between 2 Arduinos.
The most practical and cool way of sharing data from 1 Arduino to another is by far using a radio transmitter and receiver. The simplest form of wireless transmission (I could find) is the 433Mhz ASK modules. They come in pairs, a receiver and a transmitter. They are ridiculously cheap, selling at $1 or less a pair!!