PlaceKitten - Dead-easy placeholder images. Of kittens.
Lorem Picsum - Lorem Ipsum... but for photos
Fill Murray - Custom-sized placeholder images of Bill Murray
FreeImages - Find and download free stock photos - all free for personal and commercial use
Placemat - Pretty nice placeholder images.
PlaceIMG - Placeholder images for your project in progress. Custom URLs generate placeholder pictures in various sizes with categories and effects.