Put this in the "almost useless, but interesting feature" category. I recently realized that when working on a new document in most OS X applications (Text Edit, Word, and many others), the icon in the title bar of the document is slightly grayed-out until you save the document. As soon as you edit it more, it goes gray again. Additionally, if you use the Undo feature enough to get your document to match the saved version, the icon will brighten again. If you go past your last save using Undo, you can Redo until the icon brightens again.
I can't imagine a really good use for this feature except that you can quickly tell if you've saved a document since your last edit.
[
robg adds: I must admit, I'd never paid much attention to the icon in the title bar, so I hadn't noticed this behavior. I usually just glance at the red "Close" button, looking for the
black dot in its center, which is another way OS X lets you know a document has unsaved changes.]