OS X: Quick Look – preview your files
It goes without saying that by now I have become a converted. If you are running OS X, you might not know about the Quick Look function: You can preview a selected file (or series of files) without opening it with the native application. Select any number of files and hit that space-bar and the files are previewed (including movies, office-documents or zip files):
My two favorite plugins are QLColorCode (allows preview of syntax-colored source-code) and BetterZip Quick Look (allows the previewing of compressed files such as zip-files).