Flip through PDF’s in the Finder
If you hover over any multi-page PDF file within the Finder, you’ll see two arrows pop up over the documents icon. If you click these arrows, you can flip through the entire PDF right on your Macs desktop. This tip is obviously a bit more useful when your icons are displayed larger.
On a similar note, you can also play audio files and play movies directly in the Finder by hovering over them as well.
This is a fun tip that shows off the attention to detail in Mac OS X, I hope we see more in the upcoming Mac OS X 10.7 release.