I've really enjoyed reading (and playing with) kons-9's codebase in Common Lisp. It lets you do exploratory 3D modeling via the program's GUI or the repl. The source is very neatly structured and readable. I don't see many CL projects that do 3D rendering, so it's nice to see!

Quick demo:

https://www.youtube.com/watch?v=i0CwhEDAXB0

Repo:

https://github.com/kaveh808/kons-9