Mac tip: Quick Look is one of the best features introduced in Leopard, and although we've highlighted several Quick Look tricks, web site Mac OS X Hints highlights a great folder X-ray mode that's completely new to us.<!—more-->
So what does X-ray mode do for Quick Look?
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. What's more, the preview images will rotate automatically, moving through each item in the folder.
You can enable X-ray mode for Quick Look in a couple of ways. The easiest for our money is just to download and install the previously mentioned Secrets Leopard-tweaking utility, search for x-ray to find the "Show x-ray folders in Quick Look" option, and simply tick the checkbox. If you don't want to install Secrets (it can do a whole lot more than just enabling X-ray mode for Quick Look), you can simply turn on X-ray mode with a simple Terminal hack.
Fire up Terminal (/Applications/Utilities/Terminal.app) and paste the following:
defaults write com.apple.finder QLEnableXRayFolders 1 Hit Return, then Alt+right-click on Finder in your Dock and click on Relaunch. After you do, Quick Look's X-ray mode should be up and running. (Hit the spacebar when you've selected any folder in Finder to test it.) If you want an even more detailed look at what's inside a folder via Quick Look, including full directory listings, this previously mentioned Quick Look plug-in has you covered.
See folder content previews in Quick Look in 10.5 and 10.6 [Mac OS X Hints via TUAW]