I'm using org-mode, a planning, note-taking, and scheduling mode for emacs that's based on emacs' outline-mode. It provides simplified outline-folding as well as alternate views of your outline (timeline, daily agenda, next actions, etc). It uses either a single text file, or a collection of text files to do all this.

At home I keep all my projects and their actions in one text file, along with notes, calendar, unfinished blog entries, tickler file, references, inbox, and so forth. At work I do something similar, though my inbox and someday/maybe items and longer collections of notes are split off into separate files.

I find that the outline format is useful when I need to manually navigate to something, but it's not necessarily the main way I interact with my org files.

There's an org-mode tutorial with screenshots available, if you're interested. Since org files are just text, they can be used in vim, but you'll only get outline folding, not all the planning and scheduling features. I'll use less or grep to look at them when I'm in a hurry sometimes.

I think it's because viewing things hierarchically provides perspective which, more than anything else, is what I seek in an organisational system. They're much more powerful "cognitive artifacts" than flat lists, because they provide a scaffold for both horizontal (across projects) and vertical (within projects) thinking.

GOD, I have a single outline which captures my whole life. Items are also assigned a context, allowing me to view context-based to-do lists as well as the hierarchy itself. Having tried all sorts of electronic and paper approaches in the past, I wouldn't run GTD any other way. Weekly Reviews are also easier with an outline-based GTD system than any other, in my experience.

For me, the best GTD tools will always be software outliners which allow one to assign contexts to items and view context-based to-do lists:

- MyLife Organized on Windows
- Kinkless / OmniFocus on the Mac
- emacs org-mode