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.