Base 10 blocks - something that every lower elementary school teacher needs.
They're probably not economical to 3D print normally, but in some cases a teacher needs a few extras to replace those lost or borrowed and never returned.
@chuckcasey - Yeah, it's kind of shocking how much a 1000 block takes. That's why I see this as a "fill in a missing piece" rather than actually competing with manufacturers. You're not going to beat someone who is doing this on injection molds.