Comment by livrem
He also makes some (non-free) tools that are fun (says me, not a real game dev or artist).
Asset Forge is for combining 3D models into bigger models. Fun to quickly bash his various free models together to make something more complex. Also comes with a bunch of sets of building blocks. I believe he used this tool to make many of the free assets.
Kenney Shape is like a simple pixel editor, except you also set the height of each pixel and then export the result to a 3D model. Can't explain it well, but it is fun.