I admit when I first heard about FoldingText (via Brett Terpstra, I believe), I had no idea why it might be something I’d want to use. On the surface it’s a Markdown editor with foldable headers, but underneath it’s a lot more than that. You can easily drop to-do lists and timers into your document using plain-text only–in other words, you type “todo.timer” on one line, hit return, type “write for 15 minutes,” and bingo, you have a timer.
It’s still in beta, but I recommend it anyway. It’s probably the best, most efficient productivity app I’ve ever tried. If you use a Mac and find it useful to organize your work, go grab FoldingText.