Known-good everyday items to help building and testing your printer

I'm pretty sure that there is plenty of industrial goods whose dimensions are intrinsically precise because of the way they are manufactured (injection plastics, metalworking...)
Finding a good-enough cube, a trusted straight edge, and something that can be used as a set square with a flat surface would be incredibly useful for makers.
For example: with known-good plane and two cylinders of the right diameter, setting two smooth rods perfectly parallel is an easy process. Also found that cylinders are useful for checking clearances in places difficult to access with an ordinary caliper

This is my list so far:

- drill bits have correct diameter
- old VHS recorder heads are precise machined cylinders.
- hard disks (I mean the aluminium disk inside them) have fine planarity, and repeatable inner/outer diameters (at least for the set from the same drive)
- bearings have precise inner and outer diameters
- polished granite appears to be very flat. "float" glass and mirrors too, although I'm not sure (at least, it was effective spotting a a torsion in my printer acrylate piece)

I hope the list grows fast, with your help!