Placemat - Pretty nice placeholder images.
Lorem Picsum - Lorem Ipsum... but for photos
PlaceKitten - Dead-easy placeholder images. Of kittens.
FreeImages - Find and download free stock photos - all free for personal and commercial use
placeholder.pics - Lightest way to include placeholder images in your design
PlaceIMG - Placeholder images for your project in progress. Custom URLs generate placeholder pictures in various sizes with categories and effects.