Placemat - Pretty nice placeholder images.
Isometrica - 3D illustration constructor with 380+ isometric objects
PlaceKitten - Dead-easy placeholder images. Of kittens.
Characterz - Biggest diverse mix and match 3D illustration library
placeholder.pics - Lightest way to include placeholder images in your design
Khagwal 3D Illustration Library - Free 3D illustration library for web & mobile app designers