Everybody knows you can Quick Look any file in Finder by selecting a file and pressing the spacebar. Add the Option key to your shortcut for full screen Quick Look.
I like to manage my photos with the filesystem rather than iPhoto or Picasa, so this Quick Look shortcut comes in particularly handy. To Quick Look in full screen, just hold the Option key and then press the Space bar.
When you've got a group of images, video or files you want to look over, just select them all, hit Option + Space bar and arrow through the gallery in fullscreen. Not life-changing or really new, but handy to know. Thanks nettoyeur!