Ways of displaying 3D objects in 2D. Axonometry, Perspective, 6 point perspective (or 360° fisheye): (x,y,z)=>(x2,y2) And there is cyclography, too.
Try: Objector

Started as an equation plotter, now is a 2d playground.
Try: Eq plotter

Fractals created with the above playground.
For easier Julia program try: Juila

F(x, y, z) = S, Triangulization with marching cubes.
Try: Marching Cubes

Takes a curve a plane, creates unordered pairs of all points in the curve, and new point location is the midpoint, elevation is the distance.
Try: Midpoints

Sphere rolls on sphere.
Try: Spiro3

This gif is around for a while: stereographic projection of a spiral on a sphere. I had an idea to step up a dimension, 4d hypersphere projected onto 3d space. Obviously I don't see in 4d, and could made mistakes, but this program is the result.
Try: Sphere in 4D

(u,v)=>(x,y,z)
Try: Parametric surfaces

Times diagram described in a Mathologer video here
Try: Times

Voronoi diagram with interactive distance function.
Try: Voronoi

Bugs following each other. 1st -> 2nd, 2nd -> 3rd ... Nth -> 1st
Try: Bugs

Try: Pascal

(x,y) => F(x,y)
Try: planetr

Based on the cycloid drawing machine by Joe Freedman

Images recreated from a basic program in a 1984 Hungarian book 'Etűdök ​személyi számítógépekre'