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 very reliable and simple communication between microcontrollers, computers, systems, really anything with a serial port!
Point to point and multi-point networks are supported.

Note: Please check with your local laws regarding radio communication.

Features:

• 3.3V @ 50mA
• 250kbps Max data rate
• 1mW output (+0dBm)
• 300ft (100m) range
• Fully FCC certified
• 6 10-bit ADC input pins
• 8 digital IO pins
• 128-bit encryption
• Local or over-air configuration
• AT or API command set
• External Antenna Required

Documents:

• Digi XBee Page
• Datasheet
• Product Manual
• Dimensional Drawings
• Bildr Tutorial
• Digi Examples and Guides
• X-CTU Software