3b1b is really nice. I find too often that I’m all excited to be gaining so much insight to suddenly get lost at 75% of the way through. I end up having to watch them multiple times.
While we’re talking about math-related sites… numberphile is a lot of fun:
And I really like Mathologer:
It’s especially nice because he counters some of the others when they get the explanation wrong or are just confusing. And I think he’s kind of funny.
And you also can’t go wrong with Minutephysics:
Edit: oh, and I recently subscribed to standupmaths:
…after seeing him a few times on Numberphile and having bought his book (still on the ‘to read’ stack)