If you've upgraded to Leopard, you've installed a lot of handy new features. One of the handiest is QuickView. With QuickView you can look at any document, video, music, or pdf file without having to launch an application. It makes it easy to find out what something is... in an instant. In fact it's faster than OS X's own Preview app.
All you need to do is select the file and hit spacebar and up pops the quick view window. If it's a movie you can scrub through the video and if it's text document you can scroll through the text. To close, just hit the spacebar again. Remember you can not edit the file this way just preview them.