The Free Universal Construction Kit is a set of 80 adapter bricks that allows you to connect ten completely different popular children's construction toy systems. This is a groundbreaking creation.
From the product web site:
By allowing any piece to join to any other, the Kit encourages totally new forms of intercourse between otherwise closed systems—enabling radically hybrid constructive play, the creation of previously impossible designs, and ultimately, more creative opportunities for kids. As with other grassroots interoperability remedies, the Free Universal Construction Kit implements proprietary protocols in order to provide a public service unmet—or unmeetable—by corporate interests.
The Free Universal Construction Kit has adapters that connect Lego, Duplo, Fischertechnik, Gears! Gears! Gears!, K'Nex, Krinkles (Bristle Blocks), Lincoln Logs, Tinkertoys, Zometool, and Zoob! Simply incredible.
While the set cannot be ordered directly, the adapters can be downloaded from Thingiverse.com as a set of 3D models in .STL format. These can then be reproduced by personal manufacturing devices like the Makerbot 3D printer. The production of physical sets may be arranged through a personal fabrication service bureau such as Ponoko.com. You might also try a makerspace in your area. You may also find a 3D printer in the architecture, industrial design, or mechanical engineering departments of a local university. Ask nicely.