Design Principles - An open source repository of design principles and methods
Wallcat - Enjoy a new, beautiful wallpaper, every day.
Facebook Design Resources - A collection of free resources made by designers at Facebook
Splashify - Beautiful desktop wallpapers for Mac and Windows
Facebook - Connect with friends, family and other people you know. Share photos and videos, send messages and get updates.
Irvue - (previously Unsplash Wallpaper) Gorgeous wallpapers on your Mac.