3D printers are cool devices. They use resins or other materials to make real objects like parts and other things that you can use for other tasks. For instance, 3D printers can be used to print parts of a prototype design. A guy has used his 3D printer to print something that many people need - replacement house keys. I mentioned the world's smallest 3D printer a few weeks back.
The 3D printer used in the project was the RepRap 3D printer the maker also used Open SCAD to generate working house keys. To make the keys the user inputs the key code for the lock into a parametric OpenSCAD model. Then the printer will print the key and the key should actually work. I would worry that the 3D printed key might not be strong enough to survive repeated locking and unlocking.
The designer verified that the system worked by printing out keys for a box of locks bought off eBay that had Kwikset KW1 and Schlage SC1 locks inside. Those are apparently the most common locks in use today. The builder offers the files needed to create the keys as well. The only thing that bothers me about this is that it might be something that people could use to gain access to someone's home without permission.