Digi International's XBee modules are low-cost popular modules for wireless networking, and are offered in various configurations.
This module operates in the unlicensed 2.4GHz band, and is popular for implementing point-to-point or point-to-multipoint communications. Note: this product operates at 3V. If you plan to use it in a 5V system, you will need to provide voltage-level translation for safe operation.
The connector pins have 2mm spacing, making them difficult to work with, so we have created a number of handy adapters for use with this module. These include USB-to-XBee, XBee-to-RS232, XBee-to-solderless breadboard, and a 4-channel 3V/5V level-shifter breakout board. Check them out in the Adapters/Breakout Boards category, at the left.