A very nice and concise description of learning discrete functions (in this case to [approximately] reverse Conway's game of life) by approximating them with differentiable functions (Gaussians here)

A nice 20-minute video on Penrose tiling

A very useful calculator for determining win/draw/loss probabilities for two players given their ratings.

The most clear introduction to generating functions and their uses that I've found.

Fun example of using a topological theorem to solve a discrete puzzle.

On a fun way to re-discover Dirichlet's prime number theorem.

Eli Bendersky takes the derivative of a matrix.