They are probably most famous for the videos Sal does, but arguably the site is most valuable as a source of problems you can drill on, starting from the very basics if necessary. I may be biased, though, given that I used to work there!
His Recommendations page, "Other places online to explore math", a companion to his book recommendations post. He discloses in the quoted sentence that he is a former Khan Academy employee. The page carries a footnote: "Some of the companies listed above have sponsored 3Blue1Brown videos in the past, but the recommendations here are not sponsored or advertisements in any way." He does not say which companies, so every commercial learning platform on the page is labelled as a disclosed relationship.
2020-06-01·3blue1brown.com ↗