Ooh, fun! I don't have my own printer, so I have to use the ones at the library (when it's open ) or at Shapeways. Mostly what I do is dice (there's a link in the RPG Member Commercial Content thread), but lately, I've been working on a more ambitious (and much slower) project: Between 3D printing and laser-cutting, I think I can come up with a design for a real, functioning tuba, that could be made for under $100.
I've done a little plastic welding slide-vuvuzela project before, and one of the things I encountered during my research is that copper wind instruments are made of copper because of its antimicrobial properties. Plastic (probably highly dependent on the exact plastic used) could prove quite a good growing surface for quite a few different biological nasties if regularly provided with fresh spit. This wasn't much of a problem for my project, because I wasn't going to be playing it that much. (After all, referee whistles and the likes are often made from plastic, and those seem to do just fine.) But if you're thinking on tackling big tuba it might be worth taking into consideration.