We like plain text to-do lists around here, but they naturally limit you a bit. Yipgo mitigates this by keeping the text-only interface but allowing you to designate to-dos, appointments and more with certain formats. It will then compile all the important stuff into an agenda view for you.
Yipgo is the halfway point between a plain text to-do list and a more typical to-do list manager. You can fill the blank text note pages with whatever you want (and create as many separate pages as you need). If you want to designate an item as a to-do, simply preface that line with "### TODO" and anything that follows will be higlighted and added to your agenda. It's simple formatting that gets out of your way.