Further Learning

I have been using Khan for these last weeks and it is very good!
What site would Khan recommend for further learning?
Having the interactive questions style would be great too.
Brilliant seems good, but sadly it is a paid service and not an open learning place like Khan.

I have gotten mastery on all science skills and would like to learn more in-depth on all these topics. And for that matter how would this content available on Khan compare to their respective colleges?