Taskpaper is for “…Mac users who want a simpler way to stay organized and get things done. TaskPaper is a simple to-do list that’s surprisingly adept. Unlike the competition, TaskPaper’s text based interface is focused on paper-like simplicity.”

It’s cool, simple to use and quite useful. What is more useful is the vim plugin that replicates the Taskpaper idea. [Link fixed]

It’s so good, I bought a copy of Taskpaper.

P.S. If you’re going to use taskpaper.vim I would highly recommend editing your favourite colourscheme to highlight the taskpaper bits.
I use vibrantInk and added this:

    hi taskpaperDone guifg=#444444
    hi taskpaperProject guifg=#66FF00
    hi taskpaperTodo gui=NONE guifg=#F0E68C guibg=#FF00FF (Line break)
       cterm=NONE ctermfg=lightyellow ctermbg=lightmagenta
    hi taskpaperListItem guifg=#FF6600
    hi taskpaperContext guifg=#33999