Instead of just seeing the regular folder icon in Quick Look, the folder icon will be semi-transparent, with previews of the folder’s contents visible in the Quick Look window, as seen in the image at right. What’s more, the preview images will rotate automatically, moving through each item in the folder (click the image to see a movie of the animation).

To turn this on, quit the Finder (using Activity Monitor, Terminal, etc.), then open Terminal and enter this command:

$ defaults write com.apple.finder QLEnableXRayFolders 1

Relaunch the Finder, and enjoy the new folder X-Ray mode in Quick Look. If you ever want to disable it again, quit the Finder, then repeat the above Terminal command, but change the 1 to a 0.

Source: Macosxhints

Snow/Leopard – Folder content previews in Quick Look is a post from: My MacOS X