Want to do some wireless communication for the Arduino, but on the cheap? The Wicked Devices 433MHz Receiver Shield will give your Arduino-compatible ears!
You want to make this thing talk to that thing waaay over there? Try the SeeedStudio super-long range 433Mhz RF link bundle! They're claiming 2KM range for wireless data transport & remote control use!
This 315MHz RF transmitter/receiver set is a simple 100M remote-operation control link
Bluetooth! XBee! Arrgh - which way to go? Well, the Bluetooth Bee lets you jam a Bluetooth SPP module into an existing Xbee socket ed project!
This ATTiny24-based 433MHz platform is a remote sensor base. Add up to 4 sensors (analog or digital), and configure the board's broadcast schedule!