Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:1839868.

This is one of my all-time favorite construction systems. It's comprised of a single primitive. When six are joined into a cube, you have a building block that can connect on any face to any other cube. It's simple to print and amenable to injection molding. Enjoy!

Also check out Construction System 200 for a single-cube based version of this. Harder to print, but stronger and more stable.