Placemat - Pretty nice placeholder images.
PlaceKitten - Dead-easy placeholder images. Of kittens.
Fill Murray - Custom-sized placeholder images of Bill Murray
PlaceIMG - Placeholder images for your project in progress. Custom URLs generate placeholder pictures in various sizes with categories and effects.
Placehold.co - Placehold.co is a placeholding website that lets you create your desired template for image placeholding.
Plaiceholder - Beautiful pure CSS image placeholders, without the hassle.