The parts were originally made using polyurethane casting. The files uploaded here have been modified to make them printer-friendly. But I haven't tested if they actually print...
To make a part, you will need to print a top and a base. The top and base then have to be glued together. You can mix and match different tops and bases to make a variety of parts, including structural pieces, gears, racks, and so on.
SnapBlocks.zip contains 16 stl files in dimensions of millimeters
SnapBlocksAlibreSource.zip contains the source models in Alibre part format