Since I still haven't seen it in action with my own two eyes, I'm still baffled by the concept of 3D printing. In my head, I imagine it like the machine that makes those detailed masks in the Mission: Impossible movies. But it's becoming more and more popular, and cosplayers have utilized it for some incredible things so far.
Here's another awesome 3D printed piece that would work wonders for Spider-Man cosplayers: a spectacular mask created by Yuri Schuurkes from SpideyPlanet that's a replica of the one from the Sam Raimi movie. The eyes lock in place with magnets, and you can check out photos and video of the assembly below. You can buy it for yourself on Shapeways.