Flickr - Creative Commons

"Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through photos under each type of license."

Over 7,7 million images. (CC by or CC by-sa)
Sadly, this still corresponds to approximatelly only 18,8 percent of the whole set of Creative Commons licensed works at Flickr.

The rest of the Creative Commons licensed photos at flickr are non-free (aprox. 34 million images under NonCommercial and/or NoDerivs CC licenses)