This is the very popular 2.4GHz XBee module from Digi. These modules take the 802.15.4 stack (the basis for Zigbee) and wrap it into a simple to use serial command set. These modules allow a very reliable and simple communication between microcontrollers, computers, systems, really anything with a serial port! Point to point and multi-point networks are supported.
Not sure which XBee module or accessory is right for you? Check out the XBee Buying Guide!
If you are using these outside of the United States, please check with your local laws regarding radio communication.