One of the handiest features of Mac OS X for me has been Quick Look, the ability to quickly view a compatible file simply by selecting it in the Finder and hitting the Space bar.
But OS X has the annoying habit of displaying the selected graphic at an arbitrary size on your screen (as seen on the left in the image above). You can then hit the double-arrow icon in the Quick Look preview to zoom it to full screen. This is doubly annoying for anyone with a large screen because not only do you not use the full size of your screen, but you then have to move your mouse all the way down to the bottom of the screen to click the icon.
Thankfully, you can avoid all that mousing around and enjoy a full screen Quick Look preview (as seen on the right in the image above) simply by using the keyboard shortcut of Option + Space bar instead of just the Space bar.
You can also view a slideshow in full screen mode by selecting multiple files before entering Quick Look. To exit full screen Quick Look, simply hit the ESC key in the upper left of your keyboard.