What are the best sites for free stock photos?

I have been using Stock.XCHNG for the longest time.

Stock.XCHNG was launched in February 2001, as an alternative for expensive stock photography. The idea was to create a site where creative people could exchange their photos for inspiration or work. In about two years the site evolved into this massive community - there are over 1.000.000 registered users and more than 250.000 photos online!

Whether you just want to browse the huge image gallery or want to share your personal photos with others, this is the site for you!