Digi International offers a variety 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 a 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 still good enough for many applications. A good example would be the RN171 WiFi module in XBee format.
These modules are available from hundreds of stores around the world, just search the web for XBee.