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)