Khan Academy

Khan Academy is free, and not free in the sense of a trial that turns into a subscription. The entire library costs nothing, carries no advertising and has no paid tier, because it is run by a non-profit organisation. That single fact shapes what it is for. Spend an afternoon revising statistics, work out how the Krebs cycle actually functions, get ahead on next term's geometry, or prepare for an exam that is closer than you would like. Subjects run across maths, science, economics, finance, grammar, history, government and politics, among many others, with thousands of videos, articles and interactive exercises. Practice is where the learning happens rather than the watching. Exercises, quizzes and tests give instant feedback, and when you get something wrong you get a step-by-step explanation of the correct approach instead of a red cross and silence. Your progress is saved across devices, so a topic started on a laptop continues on a phone. It suits a wider range of people than the word student suggests: school pupils, teachers looking for material, home educators, adults returning to study after two decades away, and anyone who has ever wanted to understand something properly rather than skim it. Material can be worked through at your own pace and revisited as often as you like, with no deadlines and no sense of falling behind a group. That freedom is what makes the platform practical for people who study in fragments, in the evening or during a spare half hour.