My vote is for this one, I see this backpack-clad girl at least once a week, typically when I mistype a URL and end up on some domain squatter's landing page, for example http://flickr.org.
Clicking on the image will lead you to a quick and easy search with my new favorite "similar image search" tool, GazoPa from Hitachi (bye-bye TinEye!) reveals many web sites using this stock photo. That said, I was not able to find the source (Flickr, microstock sites like iStockPhoto, etc.) - anyone know where to find the original?
Do you have a contender for the Internet's most ubiquitous stock photo? Share it below!