One of the coolest innovations in 3D printing has been the RepRap Project. It's a 3D printer that can make most of its own components. Some groups hold RepRap events where interested parties can build their own RepRap 3D printer over the course of a weekend. Now NASA has taken the idea of the RepRap to a whole new level.
Everybody's favorite space agency is now working on a 3D printer that could in theory build a satellite or space station while orbiting around the Earth. It's called SpiderFab, and its goal for now is to construct "very large, very high-strength-per-mass, lattice-like structures combining both compressive and tensile elements." In short, NASA is working on a 3D printing spider robot that could construct large space structures.
NASA hopes that SpiderFab will help cut down on the weight of rockets. At the moment, most components for building structures in space are folded up inside the rocket during launch. This can add a lot of weight to the rocket and therefore make rockets burn more fuel to exit the atmosphere. With SpiderFab, they would only need the robot and the base materials for building. More importantly, it would cut down on the cost of space missions.
Since the RepRap, 3D printing has promised that self-replication is not far off. SpiderFab is not just about building, but also repair. Astronauts could easily make repairs to the ISS or other hardware in space using 3D printers instead of having NASA or another space agency send the materials to them. It's a little too early to say, but 3D printing will probably have a large impact on any potential manned mission to Mars.