If you're browsing a Stack in the Dock, you can use the arrow keys to navigate the icons. And if you press the Space Bar while an icon is selected, Quick Look appears. You can also simply hover over the item and press Space.
It's similar to the new Quick Look feature in Spotlight.
[crarko adds: Ubiquitous Quick Look is a nice thing.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110801174325472