[rckoepke]

https://runestone.academy/runestone/books/index has 100% free interactive versions of high quality textbooks. They use a JavaScript implementation of Python which runs in the browser (Skulpt) to provide a REPL.

Example Textbooks: Problem Solving with Algorithms and Data Structures using Python (3rd edition)

[vram22]

I had blogged about them, but for a different reason, also interactive - online turtle graphics:

Online turtle graphics in Python from Runestone Interactive:

https://jugad2.blogspot.com/2013/02/online-turtle-graphics-i...

Had also mentioned that anyone can host a course there for free.