https://www.khanacademy.org/

Helped me get through my engineering degree. Absolutely the best maths education I’ve ever seen.

In a very different way to most of the answers on here so far:

A Pigeon Sat on a Branch Reflecting on Existence.

It’s simultaneously excruciatingly dull and mindbendingly interesting. A genuinely fascinating film, but requires a huge amount of focus and commitment from the viewer. Possibly my favourite film ever

Had a quick scroll through and didn’t see it anywhere yet, so:

Dear Esther.

Actually, reminds me I should play through it again, it’s been a few years.