Even better is when you have the higher level outline integrated within the operating system file browser.
We don’t have a strong collection of applications that support this, but NoteTaker comes pretty close. It has a “library” of outline files. These are not displayed hierarchically, but at least there is some direct connection between the outliner panes and the existence in the Mac OSX Finder. The screenshot shows Eudora, which can be seen as an outliner of e-mail notes. In this case, the top level headers are shown as mailboxes in folders. That’s the way they really exist in the Finder.
SkinkHunt Notes has true Finder integration in one sense: notes are really individual files, openable in any text editor. Notes serve as a Finder supplement for editing and simple searching. Hog Bay Notebook is similar, but it hides its files in a Finder package.
jEdit is an open-source Java outliner that runs out of the box on OS X, but has a non-Mac interface. It is a terrific example of Finder outline integration. One of its panes is the Finder outline, like in the shot below, but the “note” is an outline. A third pane is the text of the note.