Introduced In: 10.5 "Leopard" (2007) 

Most Mac OS X users probably take Quick Look for granted, not realizing how amazing it is to click your spacebar and get a full view of the file. Want to see if the PDF you're about to click on is the right one? No problem. Want to hear a song before you add it to a project, but don't feel like firing up iTunes or QuickTime? No Problem. It's probably the feature everyone uses the most, and it's only been available for the past five years. Amazing.