I have a Buffalo WLI-U2-KG54L USB wifi dongle. It very convenient as my router and access point is placed in the basement, and I sometimes have computers on the main floor that do not have built-in wifi.
The dongle works out-of-the-box™ on Ubuntu, but as debian are more restrictive, it need a bit more work.
You can find the instructions here: http://wiki.debian.org/zd1211rw
And instructions on how to activate it here: http://wiki.debian.org/WiFi/HowToUse. The part about wpa_supplicant is most relevant, as network manager just works when the device works.