Have you thought about building an API to let websites use it with their own content ? Also, a search engine would be pretty cool ;) Instead of clicking on an item in circle #1 and then sub-section in circle #2, user could type his own keyword in circle #1 and add his own sub-section in circle #2. Obviously you can already do this on Google using + and all the others signs, but I'm pretty sure a lot of people do not know how to use it.