If you have upgraded to Apples new OS Leopard (10.5) then you have probably already used QuickLook. It is a cool utility that allows you to view the contents of a file without even opening it.

You can flip through multipage documents, watch full-screen video, even see an entire Keynote presentations, although there are some files that QuickLooks doesn't work with.

Thankfully Apple made QuickLook extendable, so developers could enhance what it is able to "Quick Look". QuickLook Plugins List and QL Plugins offer a directory listing of these. A few that I rely on are:

AI: Adobe Illustrator

EPS

FLV

ZIP: Better Zip

Folder View