It is still in mockup phase, but I will probably start building it today or tomorrow. I will use jQuery in combination with JavascriptMVC.

Well you could use it for anything where you need references and annotations near your content. For instance, you could be writing speech, and you could keep your references in comments, or you could use it for simple manager of your contacts, with each comment being a summary of your conversation. It's pretty flexible, and I want something more powerful than Notepad, but still simple to use.

One other use is writing down of martial arts techniques. You can write the technique down, add videos and pictures of technique that you found on the internet, keep log of your progress in comments etc.

Edit: I plan to create as many export formats as possible. Also you will probably be able to create public version of page.