You may be familiar with radio wiz Rick Winscot and his easyRadio modules for Arduinos. They are popular as you pop on the shield and via serial you have an instant mesh network, operating in US and EU licensed 868/915 Mhz bands. Now he's on Kickstarter with the WiPi Wireless Bridge for Raspberry Pi. It is a low current USB device that works on the Raspberry Pi and also with PC/Mac (with the standard FTDI driver).
These items are dead simple, easier than Xbee in my opinion. They also have solid antenna links and have a range of at least 300 feet, more outside/line of sight.
The Kickstarter has bundles with various USB and Arduino bundles. These will be sold later by Farnell/Element 14, CPC, Saelig, or LPRS after this offer for full retail so snap some of these up while they're available.