I’ll start with the trick that always wows everyone.
It’s called QuickLook and it’s simple – In the Finder, select a file, an image file is most spectacular, a jpg works every time. Just select it, that means single click on it, don’t double click. Now hit the spacebar. Voila, quicklook.
If you’re trying to find one file in particular in a folder of similar files, if you click ONCE on the first one, press the spacebar, then use the arrow keys on your keyboard to go up and down (in list or column view), or side to side (in icon view, as well as up and down), you can quickly display the file you’re seeking.
This works with most file formats, though not all. Works with pdfs, Word, Excel, Powerpoint, Photoshop (including most image formats), html files, text files). Out of the box it does NOT work with Illustrator or Indesign files; for those formats you need a program called Art View. Try it out http://www.code-line.com/software/artview/. If you deal in graphics files regularly the time this program will save you will be worth the $25 it costs.
Here’s a little video to demonstrate: