I just realized that VIM 7.3 has built-in support for highlighting Markdown files. Excellent. However, it doesn\'t fold on the headings.
Can any offer suggestions on
VOoM : Vim two-pane outliner is worth checking it out.
Not only does it provide basic folding, but it also provides outline navigation via a 2nd outline view pane (similar to document map in MS Word). And it supports a large number of markup languages including others mentioned in other answers - Markdown, viki, reStructuredText, vimwiki, org, and many others.
For more info see the screenshots and the help page.