Say you?ve decided to open a saved file in a given application. You press Command-O (or select File -> Open if you?re menu-inclined) and the standard file open dialog appears. You then navigate to your desired file, which is buried a few levels deep in your user?s folder. Once you?ve found the file, though, you realize you want do make a copy of it first?just in case something goes wrong with your planned edit.
You can switch to the Finder, of course, and navigate to the same folder? but that?s time consuming, and seems redundant given you?re already looking at the folder you want in the Open dialog. So here?s the trick. Select the file in question (one mouse click), then press Command-R from within the Open dialog. The Finder will spring to the foreground, open a new window, and automatically select that same file in the new window.