To start this out, here are some of my personal favorite YouTube channels:
jan Misali: Makes videos reviewing and criticizing artificially constructed languages. For example, he's praised Lord of the Rings Elvish(both kinds) and broken down what makes it so good.
: Makes videos about non-euclidian geometry in video games and vlogs his work on a hyperbolic game. He's got this crazy video about how objects appear bigger when they're further away in spherical worlds.
: Great guy that does chemistry and other cool stuff.
: Full on chemistry experiments with really good explanations.
: Electronics and basically anything involving electricity, does a really god job at explaining things.
If you'd like to share some of your own favorite YouTube channels, please do so by adding info and links to the comments in this thread.