You may already know that you can right click on any file in the Finder and choose “Open With” from the contextual menu. This gives you a list of all the apps Mac OS X thinks can open that file. An image file, for example, will show Preview (default), Firefox, Google Chrome, and any image editing app that you may have on your system, like Adobe Photoshop or Fireworks.
You may also know that tapping the space bar after clicking on any file in the Finder, Open and Save dialogues, or in Mail app, will give you an instant preview of that file. This feature is called Quick Look, and it’s been in OS X for a while, now. iTunes will play their audio content, images will zoom to their actual size, and videos, if you have the right codex on your Mac, will play in a little pop up window.
What you may not know is that these two features can be combined now in OS X Mountain Lion.