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.