You can 3D print with almost any material now.
I'm sure Staples will be using basic polymers. But, there are many other methods and materials being used now:
Sand (building molds for castings, and/or sculptures*, etc)
Metals ("selective laser sintering" as well as standard polymer deposition)
If you can imagine it, it's probably being done. Some of the methods and materials just aren't exactly cheap....
*Edited to add.
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."
Last edited by FrankenMauser; November 30, 2012 at 05:37 PM.