Looking for a Mac Outline app? Use TextEdit
I use TextEdit often for simple text editing and note taking, but you can also use TextEdit as an outliner and avoid downloading another application.
To access outline mode in TextEdit, hit Option+Tab, you can then type things quickly in outline format, then using tab and return you can work within the outline framework.
TextEdit comes pre-installed with Mac OS X, meaning this is a totally free outline solution.
Thank to Curt for sending this tip in!