If you're looking for an outliner for Linux and like VI, you should check out the VIM Outliner. It's a macro package for VIM 6 that transforms it into an outliner. Unfortunately, it still has some significant limitations, such as not saving which text areas are collapsed and which are not. But basically it's quite usable. At least it's still better than the alternatives I've found so far. Besides, you don't have to boot VIM like you do with Emacs.