Leopard includes a new Finder functionality called Quick Look, it works on most file types (movies, acrobat, html, word, excel, on and on).

Just click on a document and click on Quick Look and almost immediately a window launches with the document as it would look in its native format. So if you are looking for a .pdf file, you can click on it once, without having reader or any other Acrobat app open, and you will see exactly what the file will look like, you can even scroll through multi page documents.

It works on movies as well, the movie plays in its entirety without opening QuickTime.

When you view a picture, there is a button that allows you to move the image in to iPhoto.

This is a major timesaver as you can view anything on your mac without waiting for the native app to open.