Digi international offers a verity of wireless modules in a standard form-factor. Modules range from low power Zigbee modules, capable of mesh or point-to-point connections) to high-powered modules reaching up to 15 miles!
Another important XBee offer is the WiFi option. Some gadgeteer mainboards may not have support for the WiFi RS21 Module GHI offers. These mainboards can use the Xbee module plus XBee-WiFi as an alternative. The difference is that GHI's WiFi Module enables FEZ Spider with real WiFi connection that uses the standard .NET socket class. On the other hand, XBee WiFi is simply a WiFi to serial bridge with many limitations but till good enough for many applications.
These modules are available from hundreds of stores around the world, just search the web for XBee.
Please see Digi's website for more details.
Requires Socket type U.
.NET Gadgeteer compatible cable is included.