1. CS50 [difficulty level: medium, has certificate: Yes] https://www.edx.org/course/cs50s-introduction-to-computer-sc...

2. Algorithms [difficulty level: hard, has certificate: No] https://www.coursera.org/learn/algorithms-part1

3. Nand2Tetris [difficulty level: ok, has certificate: Yes] https://www.coursera.org/learn/build-a-computer