thingiverse
Based on a similar idea I saw recently, I have designed some stacking blocks. You can customise the top and bottom, and the words, size, etc.
I have included pre-made STL for a larger "zero" as a base, and "one" to "ten" to stack on to each other. They will generally only stack correctly. Yes, I think zero is important, software engineer and all that.
But they could be used for letters or all sorts.
The example set are printed in ColorFabb nGen apart from the brown which is ColorFabb PLA.
P.S. looks like Thingiverse don't have the Noto Emoji font - see C.stl for example of how that could have been used in customiser if they did :-)
P.P.S. My 3 year old grandson just told us it is not "zero" but "naught" - he is going places I think!
Update: March 2017 - easier stacking, and clearer letters. Also, I would suggest printing them in the colours shown - resistance is futile.
Direct link to the original creator's page
Click the "View on thingiverse" button above to visit the original model page on thingiverse. You can download the STL file directly from the creator's page for free.
This STL file is compatible with most FDM 3D printers (Creality Ender 3, Prusa MK3S+, Bambu Lab, etc.) and resin printers (Elegoo, Anycubic). Check the original page for recommended print settings and materials.
Yes, this model is available as a free download on thingiverse. Some creators accept tips or donations.
Most STL files can be modified using free software like Blender, TinkerCAD, or Meshmixer. Check the license on the original thingiverse page to see if modifications are permitted by the creator.