NoteTask. It is a fast, clean, and simple note taking application that can present your text as tasks, completed tasks, and sections/projects. By default it recognizes lines starting with hyphen and spacebar as tasks, and lines starting with exclamation mark as completed tasks. (You can customize this behavior).
There’s a free version, NoteTask Lite. It lacks Simplenote sync but everything else is the same.
NoteTask is on special holiday sale. It’s only $0.99 for limited time.