One of the most useful subtle features in OS X Mountain Lion is the ability to rename files directly from the title bar in Finder (as we noted in our “Seven Unsung Features of Mountain Lion article in the past). The feature is also supported in most apps, as well as all of Apple’s bundled applications.
To use the feature, just hover your cursor over a title bar. A small downward-pointing arrow will appear to the right of the title. Click that arrow, and the file name will be highlighted. You can now change it to whatever you like!
Update 11/25/2013: The feature also still works in OS X Mavericks, and has even been expanded – check out our updated post for more details!