Khan Academy
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Khan Academy started as Sal Khan tutoring his cousin over recorded video and grew into a full nonprofit curriculum covering math from arithmetic through calculus, plus science, economics, grammar, and official practice material built with the College Board for the SAT. Everything is free, funded by donations and grants rather than ads or a premium tier, which is unusual among platforms in this list.
Its structure suits a student following a school-like curriculum, with mastery-based progress tracking, practice exercises, and short instructional videos, more than a working adult after a narrow professional skill. It doesn't compete with Coursera or Udemy for career-focused courses, but for foundational math and science knowledge, or SAT prep, it remains close to the default free option.