OS X Mountain Lion has a semi-secret directory stuffed full of beautiful wallpapers, including photos from National Geographic and images taken with the Hubble Space Telescope. Luckily, they're easy to find and use. You just have to know where to look.
This tip comes to us from the folks at OS X Daily. To uncover these hidden wallpapers, press ⌘ + Shift + G from your desktop to bring up the "Go to folder" dialog. Then paste this in and press enter:
/System/Library/Frameworks/ScreenSaver.Framework/Versions/A/Resources/Default Collections/
Enjoy the hidden mountain wall papers :)