VimOutliner is a plugin for the Vim text editor. It provides outliner-like functionality by using text folding rules on indented lines of a text file. While not a real outliner, it is useful if you spend a lot of time in Vim anyway. Another outliner plugin for Vim is TVO,
Vim is, incidentally, one of the most powerful text editors available and completely free (open source) but with a steep learning curve.