The new Arduino Xbee shields (with the Zigbee modules from Maxstream) allow you to form one-to-one or peer-to-peer networks and are a snap to get working.
Without any configuration, you can use the Xbee as a simple wireless replacement for a USB cable, sending and receiving data with the standard Arduino serial commands. Just slip shields onto two Arduino board and you’ve got a wireless network. Or you can put the Xbee into command mode and group them into networks – for example, to have multiple boards sending sensor data through a central Arduino to the computer. The Xbee module can transmit up to 100 feet indoors or 300 feet outdoors (with an unbroken line-of-sight). Replace it with an Xbee pro for outdoor communication over as much as a mile. All the pins of the Xbee module are broken out to allow use of the advanced features of the board. The complete schematics and design files for the boards are available under a Creative Commons Attribution Share-Alike license from the Libelium Squidbee wiki download page. You can order fully-assembled Xbee shields from from PCB Europe (in Italy) or from Libelium (in Spain).