I love Quick Look. Especially with images. Go to an image (I've tested JPGs and PSDs so far), and hit the Space Bar. A large version shows up like it's supposed to.

Now, hold down the Option key and click somewhere on the Quick Look image -- it zooms in! Hold Shift-Option, click, and it'll zoom out. When zoomed in, release all keys and click and drag -- you can move within the zoomed image. That, my friends, is greatness.

With my unscientific testing, the zoomed in Quick Look image has just a hair less clarity compared to the same image zoomed in using a graphics program -- but nothing to gripe about!

[robg adds: You can also zoom in and out by holding Option and using the scroll wheel on your mouse, or two-finger scrolling on a trackpad. This technique was mentioned in the comments to this hint on zooming PDFs, but the above adds more details.]