Lucas 3/19/2013 12:02 am
The best solution I've found for plain text outlining is to use the crossplatform (Java-based) text editor jEdit.

http://www.jedit.org/

Go to settings, click on 'Editing', then for 'Folding Mode' select 'indent'. And for 'Automatic indentation', select 'full'. That way, text folding is activated on the basis of indentation. To indent, you press tab, and then the next line will automatically be indented the same amount. A 'Folding' menu in the menubar allows for collapsing and expanding all folds.

Screenshot with 1 line folded:

http://goo.gl/NdRgn

Screenshot with same line unfolded:

http://goo.gl/tPgLW

Basic outlining is achieved in this way, although without any special features.

A far geekier but probably superior solution is this forked version of Vim Outliner:

http://www.vim.org/scripts/script.php?script_id=4223

(Vim itself also allows for indent-based folding, as in jEdit, but Vim has more of a learning curve.)