Scribe is an easy outliner for the Mac.
December 5, 2011: The application works very well. You just need tab and delete to move headings around. You can paste the outline into TextEdit and the formatting stays intact. However, saving it as RTF file and opening it in MS-Word leads to rather unpredictable results. Importing or pasting it into Mellel works better, though. Hitting tab at the beginning of each item, makes them line up correctly, so the outline structure remains intact.
It would be good, if it could save to OPML or other formats.