We use a time based currency: Students are required to pass on their learning in order to earn credits that they can use to take further free courses.
Knowing how to teach a subject and answer questions from other students is a great way to master a topic. Each participant is a student and a teacher, which allows to keep small class size and keep learning personalised.
Knowledge experts and companies volunteer time to support this work by volunteering teaching time. They know that for every hour they donate is matched by students time supporting the learning experience.