Dave Metzener's Weblog

macosxhints.com – See folder content previews in Quick Look in 10.5 and 10.6: “Quick Look has a hidden feature that is not enabled by default, and can only be enabled through Terminal. Essentially, when ‘X-Ray mode’ is turned on, Quick Looking a folder will show, in a way, the contents of the folder.

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. “

This is one of the best tips/tricks I have run across is a long time. Not only do you get to see thumbs of whats in the folder, but they cycle through all the files. New image start at the top and slide under as new images arrive. Its a great effect and something I am going to keep on my machnes.