One of the cool things I just discovered on my Mac is the slideshow capability of the Finder. This can be a great tool for previewing a collection of images, or just to rotate images on your screen just for fun. Just highlight a few images, right-click, and choose the Slideshow option.
The images are then displayed in full screen mode, one after another, and keep looping until you hit the [Esc] key. Moving the mouse is another way to stop the slideshow, and even offers other options, such as showing the images in an "index sheet", letting you choose a file to view, or sending the images to iPhoto.