Khan Academy

Khan Academy organizes knowledge as a dependency graph rather than a course list: skills connect into units, units into subjects, and a color-coded mastery dashboard shows exactly where you are solid and where you are shaky. Practice problems are the core loop, hint-laddered so a stuck learner can reveal one nudge at a time instead of jumping straight to the answer. The signature visual is the mastery bar shifting from blue toward dark blue as you prove retention through spaced review, not just a single correct attempt. The interface is famously plain, almost utilitarian, which keeps focus on the math and the explanation. The weakness is breadth-induced sprawl: the sheer depth of the tree can leave a self-directed learner unsure where to start.