Mathigon is an interactive online textbook about maths. It covers geometry, algebra, and probability and discrete mathematics.
Rather than just presenting you with information, Mathigon challenges you with problems and allows you to interact with diagrams. It is one of the most accessible and engaging maths resources available on the web, a true mathematical wonderland. The text is well-written, the pages are beautifully designed and presented, and it covers a large array of topics.