Open Music Theory is a free, interactive text book
Open Music Theory is completely free and interactive. It goes a little deeper than musictheory.net and covers just about everything you could expect to learn in an undergraduate core theory course and more. What I really love about this site is that they cover more than just 18th century traditional theory. They have discussions on form in pop and rock music as well as poetry in music!
Here’s a screenshot of one of their lessons on the doo-wop chord progression:
It’s a perfect, free way to start learning core music theory.
They’ve also developed a more formal, interactive textbook: Open Music Theory Version 2 (OMT2)