Mac OS X has a fabulous feature built-in for previewing files.  It's so easy to use it's ridiculous 🙂  In the Finder, locate any file that you want to preview.  Click it once and press the Spacebar.  This is known as QuickLook and it will instantly open a window in which you can see a preview of the contents of the document.

This feature has now also been integrated into the Preview pane which appears when you select a file in the Finder.  For any supported file, which is most files such as images, movies, Word documents, Pages documents, PDFs etc., a preview will appear in which you can either flick through the pages of a document using the arrows provided, or even watch a movie file by pressing the play button.

QuickLook Preview

This allows you to quickly check if it's the document you're looking for, or watch a quick clip from a movie to be sure it's what you want.  It's very clever and very fast, hence the name QuickLook...