1. Cloud natural language API.
https://cloud.google.com/natural-language/docs
2. Edx.
https://www.edx.org
3. The great noun list.
http://www.desiquintans.com/nounlist
4. Khan academy.
https://www.khanacademy.org
5. Ally, M.: Foundations of educational theory for online learning. Theory Pract. Online Learn. 2, 15–44 (2004)