Here’s a quick tip for Quick Look.

Ordinarily you enter Quick Look by selecting a file then hitting the space bar. If you want to view the preview in full screen, you can then click on the Full Screen button at the bottom of the Quick Look window.

A quick way to jump straight to full screen is to press Option-Space instead of Space with your chosen file selected. This will instantly bring up a full screen preview of the document. To exit, just hit the Escape key. Unfortunately this only works in Snow Leopard, but you can get the same effect in Leopard by pressing Command-Option-Y.

I find this is especially useful for quickly reading PDF documents, which are often too small in a normal Quick Look window. Also, if you select multiple images and then press Option-Space, you can quickly start a full screen slideshow.