Here's a great trick from Macosxhints.com -- set up your Mac to automatically preview all downloads with Quick Look.
It's a simple two-step process. First, install the Quick Look Droplet, a simple application that displays any file with Quick Look. Next, set your browser preference to automatically open certain file types (say, PDFs, Word documents and JPGs) with the droplet. It's much snappier than launching Preview or Word.