When you're working in an OS X application such as SweetMail, HTMLEdit, or TextEdit (and others, I'd expect) there's a subtle but useful visual marker to denote unsaved changes.
When you've made changes that are not yet saved, the red document close button will have a small black dot in its center; when you save the document, the dot vanishes.
Quite handy as a quick visual method of checking whether you've saved your latest changes or not.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20001231153805100