These are the first 3D printed ball and socket joints we've seen:
There is surprisingly little information available about such mechanisms that people have built using their 3D printers. In particular, relatively few people seem to have investigated 3D printing plastic parts that snap together. Surprising given that the main practical application of ABS plastic is Lego!
PolyBot is the first tetrahedral delta robot we've seen:
Tetrahedra are not mechanisms so they are inherently stronger than the more common triangular prisms and cubes and, consequently, don't require the same amount of steel rod which is expensive to buy and even more expensive to ship.
Novel designs that leverage the advantages of 3D printing are perhaps the most interesting. Emmett Lalish has kindly open sourced his design for 3D printed bearings that use herringbone planetary gears:
This design is particularly interesting because the parts are printed in-place and cannot escape once fully constructed.