Alamy – one of the oldest Stock Photos site – contains the largest collection of stock photos. Free photos marked as RF (Royalty Free).
Bigstockphoto contains a section "Royalty Free Photos". Photos in this section are available for free use and divided into categories.
Everystockphoto contains more than 3 million free photos from various sources. Free photos have a license: to Share and to Remix.
Corbis offers an excellent collection of free stock photos. The site has user-friendly interface and is easy to search. Free photos are licensed – RF.
User-friendly site with free pictures in big sizes. If you want to download photos in large size you have to register.
For downloading free photos from this site you need to register.
ImageBase provides completely free photos for noncommercial and commercial use. All photos are classified under several categories.
Stockvault is a good website with lots of free stock photos. All photos are available for downloading in large sizes without prior registration.
Freepixels contains many excellent free photos. All photos are in large size and free of charge.
Another site containing only free stock photos.
Regulary updated website with free photos. To download photos you have to pre-register on the site
PixelPerfectDigital is a great collection of free photos. On the site you can also download free backgrounds, 3D rendered images, skies, etc.
FreeDigitalPhotos contains high quality photos. You can download these photos free of charge in small size (640 x 425 px). The photos in large sizes – are paid.
Dreamstime contains more than 6 million images available for free download. To download the photo you have to register.
Indisain provides high quality free images and wallpapers. Requires registration.
Also check the list of websites where you could find great wordpress themes.