I built an interactive Music Theory course 8 years ago over a winter break and it continues to bring in enough to pay my rent each month. I just thought there had to be a more intuitive way to learn music theory than the very boring and jargon-heavy alternatives. It uses Tone.js to include little interactive pianos, guitars, and other demos. I've done no marketing, it hit the HN front page for a day, and after that initial spike in traffic has been fairly consistent over the past 8 years. It uses Stripe for payments and for the first few years it was only Stripe. 3 years in I decided to add PayPal support... revenue doubled overnight, mostly from international customers. https://www.lightnote.co/