The most common material used is ABS plastic, but there are a wide range of materials used.
I doubt it would be suited to anything requiring ultimate ridgity, but things like focus masks, lens caps, eyepiece caps, trays, knobs, even gears can be easily made.
I've seen the emblem robotics kit first hand, it's made from printed components, even some of the gears are printed.
Here's some background info.