The Finder's Get Info window (View > Get Info or cmd-i) is one of those unsung heroes of Mac OS X computing. It houses a lot of great options, and Stationery Pad is one of them. If you have any kind of workflow where you need to repeatedly modify some sort of a file template, checking this option in any file's Get Info window will tell its parent application to open a copy of it by default, ensuring that the original file remains unedited. While some applications that manage files for you can offer similar functionality, this is a great option for anyone who, for example, use Word docs as contract templates, or begins with a basic pre-built file for various Photoshop or hand-coding CSS/PHP freelance work. You can simply build the file any way you like, save it and check this box in Get Info to tell any app you open it with to open a copy instead of the original.

Easy breezy file templates, with no AppleScripts or plugins required.