CS50
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CS50 is a single course, not a platform, but it's substantial enough and popular enough to stand on its own: a full-semester introduction to computer science covering C, Python, SQL, and web development, filmed from Malan's actual Harvard lectures and released free on Harvard's own site and YouTube, alongside a large community of students helping each other online.
The free version gives you everything, lectures, problem sets, and solutions to check your work, while the edX-hosted Verified track adds a Harvard-branded certificate for a fee and access to some additional support. It fits a genuine beginner who wants a demanding, well-taught foundation in programming and computer science thinking, more than someone after a quick, narrow skill like a specific framework or tool.