Print the missing links...

How A Geek Dad And His 3D Printer Aim To Liberate Legos - Forbes

''In March Levin and his former ­student Shawn Sims released a set of digital blueprints that a 3-D printer can use to create more than 45 plastic objects, each of which provides the missing interface between pieces from toy construction sets. They call it the Free Universal Construction Kit.'' [source]

See them all here: http://media.fffff.at/free-universal-construction-kit/images/free-universal-construction-kit-poster.pdf