Using a pretty old 3D printer and $30 worth of plastic, AR-15.com user HaveBlue reported building his own rifle lower receiver, the portion of a weapon considered "the gun" because it is the area where bullets are struck. He modified an existing plan by strengthening both the takedown lugs and bolt hold lugs while adding an integral trigger guard, to ensure sturdier functionality -- you can download online the modified plans.
HaveBlue's site is currently down probably from the attention this is suddenly receiving; however, from The Next Web and Extreme Tech you can gather the most details and see some of the images originally posted on his site:
Before you go about locking yourself in your closet, you should know that the only printed part of the gun was the lower receiver. But, according to the American Gun Control Act, the receiver is what counts as the firearm.
HaveBlue reportedly used a Stratasys 3D printer to craft the part, assembled it as a .22 pistol and fired more than 200 rounds with it.