Much as I'd like to have Neil making sexy time with a Range Rover as my desktop wallpaper, on Windows I actually use Bing Desktop, which gives me a lovely high res new photo from around the world every day. Today is Alpacas:
On the Mac, I have this OS X Mavericks wallpaper at the moment:
Although I quite often have a realtime Earth / weather thing going on:
You may have noticed I am super anal about not having any icons on the desktop (except the Recycle Bin).
iPhone has the standard iOS 7 pastel bokeh effect animated wallpaper: