We previousely built something called as Qlazzy.com.
Goal of Qlazzy was to be a central repository of all open educational resources with easy accessibility and most features required to master that content. One the features was Notes where people could Add Learnings to an open lesson.
As you can see learners could add notes to the lesson on the left.
We noticed that the learners who added or interacted with the notes were using the product 3-6 times as compared to other features.
During college I bought notes from a senior and they were extremely useful.
So, we decided to launch Classmint which is Study Notes product.