This isn't the most useful tip in the world, but it's kind of cool, especially if you like making your Mac work just a little but differently than most...

You can enable an "X-Ray" mode for Folders in Quicklook. When this mode is enabled, instead of just showing a folder icon when you Quicklook a folder, you see some indication of the type of files in the Folder. Here's an example of what the movies Folder looks like with and without this option enabled.

Quicklook of Folder WITHOUT X-Ray mode

Quicklook of Folder WITH X-Ray mode

It's not going to change your life. But it is showing the actual thumbnail images of the movies which is kind of cool. It works on more than just movies too. For recognized file types, it''ll show the appropriate image or thumbnail of the file.

To enable this X-Ray mode in Quicklook, open a terminal window and run this command:

defaults write com.apple.finder QLEnableXRayFolders 1

That's all there is to it. If you decide you don't like this mode, you can run the command again and change the 1 to a 0 to get back to the "regular" mode.