This is the lower receiver of an AR-15 rifle, printed in fused plastic filament from a digital model that was, until this week, freely available for download on Thingiverse.
This part is significant because all other parts of the common rifle can be readily purchased in the open market. A person who builds a working lower receiver has, in the eyes of the state, essentially built a working AR-15. It is legal to do so for personal use (at least under US law), but until lately the required tools, time, and talent put the project beyond the reach of most casual tinkerers.
The rise of desktop manufacturing, however, may be set to change all that. Recently, a 3D printed AR-15 lower receiver made of fused plastic filament was demonstrated to fire and cycle six times before breaking.
Up to now, however, the policy has gone largely unenforced.
From the comments: Sean Ragan on December 20th, 2012 at 6:56 pm said:
I note, as an aside, that it just took me about 3 minutes to nab a torrent containing digital models of an AR-15 lower (and the parts for a 5-round magazine) from The Pirate Bay’s “physibles” section.