http://gigaom.com/apple/ten-easter-eggs-to-find-in-your-mac-os-and-applications/

My favourite:

Enable ‘X-Ray Folders’ in QuickLook

The QuickLook feature of Finder is great, but if you use it on a folder, you won’t see anything except a folder icon. Using this hidden setting, you’ll be able to see the contents of the folder when you use QuickLook.

defaults write com.apple.finder QLEnableXRayFolders 1 && killall Finder

To turn off, replace the ’1′ with a ’0′.