The Free Universal Construction Kit (which we are taking great pains not to abbreviate anywhere in this article and will henceforth refer to it as "the Kit"), is a set of about 80 adapter blocks that can be used to connect all of those childhood construction sets you used to play with (Legos, Tinkertoys, etc.) to one another. And it's about freakin' time, too.
I dunno about you, but when I was a kid, I relied on friction, gravity, brute force, rubber bands, telekenesis, death threats and occasionally even superglue to convince my Lego sets to play nice with other construction toys. "The Kit" (as it is known around here) contains free plans for almost 80 different 3D printed adapter blocks that can be used to interconnect any combination of Lego, Duplo, Fischertechnik, Gears! Gears! Gears!, K'Nex, Krinkles (Bristle Blocks), Lincoln Logs, Tinkertoys, Zome and/or Zoob. It's pretty brilliant.
All you need to implement this stuff is a household 3D printer (or the willingness to rent some time on one), and a kid (or adult) with the desire to Frankenstein together a bunch of different construction sets to make something improbably cool. Also included are plans for one single universal adapter block that lets you stick all of the above together all at once.
One potential pitfall to this whole system is that most 3D printers don't quite have the resolution required to perfectly duplicate the precision with which pieces like Legos snap together. Lego bricks interact with each other so smoothly thanks to manufacturing tolerances on the order of 10 microns, while most affordable personal 3D printers (like Makerbots) have an X-Y resolution of about 100 microns and a layer thickness almost four times worse than that.
The upshot here is that there's still some hardware advancement necessary before we'll all be pumping perfect copies of F.U.C.- er, The Kit adapter pieces out at home, but we're getting there, and if you set your little tyke at some rough pieces with sandpaper and calipers, everything will end up fitting together perfectly anyway.
See an overview of the Free Universal Construction Kit in the video below.