Khan Academy

Khan Academy remains the gold standard if you want serious learning without paying a cent. It’s a nonprofit with thousands of free lessons and practice exercises in math, science, economics, finance, history, and more, plus test prep for SAT, LSAT, and other exams.

The magic is in the mastery system. As you work through exercises, the platform tracks how well you understand each concept and serves up questions that target your weak spots. You get a dashboard, progress tracking, and clear recommendations about what to review next—very similar to having a patient tutor that never gets tired.

If your goal is to build a rock-solid foundation in algebra, statistics, or basic programming before taking on paid certificates, Khan Academy is where you start. It’s also perfect if you’re going back to school after a break and need to refresh the essentials.