Lorem Picsum - Lorem Ipsum... but for photos
PlaceBear - Bear-themed placeholder images for developers
FreeImages - Find and download free stock photos - all free for personal and commercial use
PlaceKitten - Dead-easy placeholder images. Of kittens.
PlaceIMG - Placeholder images for your project in progress. Custom URLs generate placeholder pictures in various sizes with categories and effects.
LoremFlickr - LoremFlickr is a service that provides free placeholder images for web and print.