This began in my earliest days with a 3D printer and OpenScad. I've updated the original code to Version 2 which does a better job of laying out the spacers on the print bed. They will also print a little faster than before because of the reduced print-head travel.

Originally I needed some spacers to create an air gap for cooling a Raspberry Pi being mounted behind a unit. I'd printed one of these cases which, as the designer comments, won't actually let air through the bottom slots - https://www.thingiverse.com/thing:4785712
Searching on Thingiverse didn't quickly turn up something suitable, so I put together a simple design in openSCAD.

It's easy to change the parameters in the .scad file to make spacers that will fit any specific application. Just set the parameters for your target outside diameter, inside diameter and height, and the number of spacers you want. 
The customizer in OpenScad (Window / Customizer) makes it easy to see what you'll get.
But beware - if you put strange values in, you'll get strange results out!

The .stl files will print spacers as described in the file names. 
For example: “standoff 4 x w10 h8 hole 4.25.stl” will print 4 spacers, each 10mm outside diameter, 8mm high, 4.25mm internal diameter (to fit over M4 bolts).