Khan Academy used to let you watch videos, do questions, and collect points. Then you would sign in to claim them.
Their new homepage looks to push you to sign up first though. There is still a link to "Unclaimed points > Sign up to claim your points" in the top nav.
"Want to save your work? Create an account."
Creating an account can use FB/Google, or local login.
First demonstrate value, and people will gladly save their work.