Here's a neat trick if you want to zoom in on an image while using Quick Look. Hold down the Option key while your mouse is hovered over the image, and it will change to a magnifying glass. Once you've zoomed in, drag the cursor, use the scroll wheel, or two fingers on a trackpad to move around the image. Press Option-Shift, and then click on the picture to zoom out.
To see this tip in action, check out the brief video above. Nothing too complicated, but definitely a fun and easy way, built directly into the OS, to browse around and inspect a picture file without opening a much heftier app like Photoshop.
[Via Mac OS X Hints]