Zooming in Quick Look!
This is a Leopard only tip. There is no obvious way of zooming in when checking out a pdf or an image in Quick Look. It is possible however. To do so, simply open a pdf or an image in Quick Look, hold down "Option" and click anywhere on the file. To zoom out, use "Shift - Option - Click". It also works in full screen. It is very useful because I thought at first that there was no way to see it bigger than full screen.