Color Palette Generator - Enter the URL of an image and find its color palette
Placemat - Pretty nice placeholder images.
Color Hunt - Curated collection of beautiful colors, updated daily
PlaceKitten - Dead-easy placeholder images. Of kittens.
Coolors.co - The super fast color schemes generator! Create, save and share perfect palettes in seconds!
placeholder.pics - Lightest way to include placeholder images in your design