Placemat - Pretty nice placeholder images.
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.
placeholder.pics - Lightest way to include placeholder images in your design
Placehold.co - Placehold.co is a placeholding website that lets you create your desired template for image placeholding.
Bacon Mockup - Use pictures of tasty meat as placeholders in your design