PlaceIMG - Placeholder images for your project in progress. Custom URLs generate placeholder pictures in various sizes with categories and effects.
Dummy Image - Dynamic Dummy Image Generator
Placeholder.com - Placeholder.com generates custom placeholder images on the fly.
Lorem Picsum - Lorem Ipsum... but for photos
Placemat - Pretty nice placeholder images.
LoremFlickr - LoremFlickr is a service that provides free placeholder images for web and print.