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
Fill Murray - Custom-sized placeholder images of Bill Murray
Unsplash - Unsplash is a website with high-quality free HD images. It has a catalog of more than three hundred thousand striking images that are neatly organized with tags. Read more about Unsplash.
Placemat - Pretty nice placeholder images.