UPDATED : July 2013
Since writing the original post 2 years ago, WWO have now included all of my icons on their site at http://www.worldweatheronline.com/api/docs/weather-icons.aspx. I’ve also updated many of the sets to a massive 92 weather conditions to make them (hopefully) some of the most accurate icons available anywhere, that now more closely reflect the actual weather condition being reported.
Visit my store for the full range of Weather Icons