The Raspberry Pi is a $35 single-board ARM computer that runs Linux. If you don't yet have a Raspberry Pi, you can either order Raspberry Pi from Newark/Element 14 and get on in line waiting for the next batch or pay way too much for a Raspberry Pi from Amazon.com.
Here are two nano-sized, low-cost, USB WiFi adapters that I have verified work on a Raspberry Pi ("Wheezy") right out of the box.
The AirLink101 AWLL5088 Wireless N (
0bda:8176 RTL8188CUS 802.11n WLAN Adapter) is shown on the left and the Edimax EW-7811Un Wireless N (
7392:7811 EW-7811Un 802.11n Wireless Adapter Realtek RTL8188CUS) is shown on the right.
Both of these WiFi adapters are based on the Realtek RTL8188CUS chipset. Drivers are built-in with Raspberry Pi Raspbian ("Wheezy"), the debian-based distro used by most beginners.
Not only are they based on the same WiFi chipset, they are virtually identical in size too. These are "nano" sized which means they're not much larger than the USB connection itself.
Since the Raspberry Pi only has 2 USB ports, you're most likely going to want to get a powered USB hub as well. That will allow for a mouse, keyboard, and the WiFi adapter with additional ports available for more devices.