[Emphere]

I would like to second this recommendation. For a long time I thought their content was low quality edutainment because of the generally 90s website design and the generic stock photos used for courses(admittedly, that's...a pretty shallow line of reasoning). The courses are usually taught by university professors, have a well thought out lesson plan and have helpful accompanying guidebooks (you can also request course transcripts, I believe). For example, the course linked above is taught by Paul Zeitz based on his book, "The Art and Craft of Problem Solving", which is a classic for learning Olympiad-level problem solving techniques. I've also learned a lot from their history courses.