FloatNotes is a Firefox add-on with which you can create sticky notes on nearly any website.
Felix Kling started to write this add-on as personal project, partly to support him in his studies and partly to learn how to develop add-ons for Firefox. There already have been a couple of other add-ons like that. But they did not meet his requirements:
And what do programmers do if they cannot find the perfect program? They write their own :)