Splashy - Breathtaking, auto changing wallpaper app for macOS, Windows, Linux, iOS & Android
Wallhaven - Your source for the best high quality wallpapers on the Net!
Irvue - (previously Unsplash Wallpaper) Gorgeous wallpapers on your Mac.
Picky Wallpapers - Picky Wallpapers is a good website that has the selection of HD quality wallpapers only.
Splashify - Beautiful desktop wallpapers for Mac and Windows
InterfaceLIFT - High-resolution widescreen wallpapers, desktop icons, and themes for Mac OS X, Microsoft Windows...