If you have an RPi and fancy WiFi then an Asus USB N-10 will do. You’ll need to manually set it up. To save anyone else the pain, the steps to get the dongle up and running on Debian are:
wget -O r8712u_ko.zip http://goo.gl/7ZYhZ
cp r8712u*.zip / && cd /&& unzip r8712u*.zip
From here on you can configure it for your network as normal.
Update: thanks to Ben for emailing me and pointing out the lack of -O for the wget line which made the remaining commands fail due to the download being saved as 7ZYhZ! So much for ninja-edits to shorten the original URL!