Material Capture, or "matcap" for short, is a really neat technique, and now that I've learned about it, I'm pretty surprised that it doesn't come up more often in gamedev conversations. It's great! Here's a pico8 implementation of the concept: it draws approximated realtime reflections on 3D meshes.
There are three settings that you can change while it's running:
Right/Left: Switch materials
Up/Down: Fast-mode / Slow-mode
O/X: Switch models
Fast-mode is kind of like a vertex shader in modern 3D engines (the material texture is only sampled along the edges of triangles). Slow-mode is kind of like a pixel shader (except there's no GPU to give it a speed boost)