Area of sphere - strangely beautiful in its simplicity

MetaPost + ConTeXt rendition of Oliver Byrne's "The first six books of the Elements of Euclid"

hyperbolic - Python 3 library for generating hyperbolic geometry and drawing it.

Topos Theory course

Discrete Differential Geometry Forum

Discrete Differential Geometry course by Keenan Crane (2020)

GeoGebra - Dynamic mathematics software.

Polyhedra Viewer - Explore the relationships between convex regular-faced polyhedra. (Code)

Twenty Proofs of Euler's Formula: V-E+F=2 (HN)

Qhull - Computes the convex hull, Delaunay triangulation, Voronoi diagram, halfspace intersection about a point, furthest-site Delaunay triangulation, and furthest-site Voronoi diagram. (Code)

Non-Euclidean Geometry Explained (2020)