LilyView is a lightweight, Mutli-Touch friendly image viewer that's handy and doesn't get in your way.
Lightweight. There's no image library to load, you just browse images in a folder.
Chromeless. No controls are visible until they are really necessary.
Multi-Touch friendly. You can literally touch the image when using a trackpad, but LilyView works just fine with keyboard and mouse.
And it's only the beginning...
Requirements: MacOSX 10.7 or later, 64-bit processor