I'd be careful about creating these types of apps because a lot of people (especially developers) value privacy.

What your describing is monitoring everything I do on my computer and then likely transmit it to your site. Even if it didn't transmit anything I would still be reluctant to use it unless it were open source.

To detect whether or not you're working on what you're supposed to be working on, you'll probably end up recording what window is focused every few seconds right? You might want to hack together an auto hotkey script (Windows) to do this. With a bit of AHK knowledge you could probably write a prototype in a day.