Freepik

A great site with resources for designers (icons, photos, stock images, etc.). There is a freemium model, and you can pay for access to more content, but there are thousands of open assets here.

http://www.freepik.com/

“Freepik offers users, high quality graphic designs: exclusive illustrations and graphic resources carefully selected by our design team in order to provide our users with great content that can be used in both personal and commercial projects.”