Specifically, I'd like to use a platform that:
1.) Allows you to teach the course completely for free
and, perhaps most importantly since most offer the above,
2.) Isn't terribly restrictive. I understand that these platforms have a business model and thus try to keep teachers and students "locked in" to their ecosystem, e.g., disallowing outside communication. I'd like to teach a free course where students would be able to contact me in whatever manner they wish, whether that's via the platform, email, Skype, and so on.