Folding Text (US$14.99) is a handy Markdown-based editing app for the Mac. Brought to you by the developers of WriteRoom and TaskPaper, it displays Hog Bay Software's commitment to clean interface design supporting strong text features.
To say Folding Text is a to-do list editor that allows you to show and hide sections is to miss a great deal of the application's nuance. Its enormous feature set makes me think that the people who develop this tool also use this tool, and are growing it to accommodate real-world practical needs.