The Free Universal Construction Kit is a set of 3D-printable adapters that enable interoperability between 10 popular construction toys, including Lego, Tinkertoys, and Lincoln Logs. The kit allows children to combine different toys and create new forms of constructive play. The project aims to promote creativity and extend the value of these toys across a child's life. The adapters can be downloaded and printed using a 3D printer, and the project is licensed under a Creative Commons license.