The Universal Construction Kit is a set of open source components that you can fab on a 3D printer, which allows the user to hook together different construction kits without duct tape or twist-ties. The components make it possible to use Lego, Duplo, K'Nex, Lincoln Logs, and other construction kits in the same project simultaneously.
The whole set can be downloaded as a .zip file from the website.
The .stl files carry a Creative Commons By Attribution/Noncommercial/Share Alike v3.0 license.
Note that the .stl files are in inches, not mm. Double-check the settings of your 3D printer before fabbing!