Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:1842366.

This is another one of my favorite building block systems. This one-piece cube connects any face to any other face at any orientation. It is similar to Construction System 197, but prints with fewer parts. The downside is that it's harder to 3d print. Some supports are necessary. The upside is that it is, generally, stronger and more stable at scale.