If any high schooler is falling behind in class, missing specific concepts or looking to improve their SAT score, they know to go to Khan Academy.

In 2004, Salman Khan, a hedge fund analyst, began creating math tutorials for his cousins. Those videos eventually turned into Khan Academy, a non-profit learning website with free, high-quality learning modules and videos for almost every high school class, including Advanced Placement (AP) classes.

In the past, tutoring, test prep and materials that help students improve their grades and test scores came with a hefty price tag. The Princeton Review usually charges $899 for their cheapest SAT prep course, and even local companies, such as Atlanta-area Edison Prep, charge $679 for an 18-hour group SAT or ACT class. By making these resources free, Khan Academy is revolutionizing learning and test prep.

Khan Academy’s simplicity and intuitive user interface make it incredibly straightforward to navigate for learners of all ages. But the reason so many students and teachers love Khan Academy isn’t that it is easy to use. It’s because it really does help you learn. Khan himself teaches the bulk of the thousands of videos, and he’s a fantastic teacher. Concepts, even complex ones, are explained in a simple, step-by-step manner and with plenty of examples (in Khan’s soothing voice) that leaves the viewer with no questions when the video finishes.

If the learner does have questions, they can also do practice modules, which test their knowledge of the concepts. They can take unit tests, whole course assessments or quizzes on specific material. For every question missed, there’s a simple explanation that helps learners not to miss similar questions again. This seems like it should happen on every learning platform, but it doesn’t. The pure effectiveness of Khan Academy is astounding from a student’s perspective, as other sites just aren’t that good.

Take AP Classroom: a learning platform run by the College Board that seeks to provide students with better preparation for their AP tests through College Board-approved videos and quizzes. Unlike Khan Academy, it’s hard to navigate, requires a teacher to unlock materials (students sometimes aren’t able to obtain the resources at all) and contains lackluster or non-existent question explanations.

Through a partnership with the College Board, Khan Academy also offers free SAT and PSAT prep. Students can take an official SAT practice test on the website, have it scored with a personalized study plan designed to fix weaknesses in seconds. A study by Khan Academy says students who used their practice for six or more hours and engaged in at least one best practice behavior (leveling up skills, taking a full-length test or following practice recommendations) scored 39 points more on the real test than students who didn’t.

Khan Academy really does help students learn. A study from the University of Massachusetts-Amherst showed an app from Khan Academy gave children from low-income families “substantial gains in their pre-literacy skills that brought them nearly to the national average” and that the app has the potential to “help level the playing field.” It doesn’t just help children, either. At a series of community colleges in New England, Khan Academy was shown to help students taking remedial or developmental math courses “meet success.”

It’s amazing that a simple website can spark these results. Free, quality resources are few and far between, and Khan Academy has shown you really can be successful in school and beyond without paying loads of money. To all the students getting ready to shell out money on AP prep books for the upcoming tests: check Khan Academy first. The best resources may be dangling in front of you already.