An OpenSCAD module for writing variable-width text. Download Spiff.scad andspiffsans.scad, then see the Instructions for how to use this in your own works (or usepoorman-spiff to generate a copy-pasteable demo). 
Spiff.scad on GitHub

Requirements

Spiff.scad requires at least OpenSCAD 2013.06, as it uses module recursion to calculate variable glyph widths. 
If you're stuck on an older version of OpenSCAD, you can still generate the equivalent code for writing text using poorman-spiff.

Due to OpenSCAD's recursion limit, you may encounter problems writing longer strings. If this happens, try breaking your write call up into multiple calls with shorter strings.

Note that, while this is listed as a derivative of Write.scad, it is not derivative code, and is almost completely incompatible (they're almost compatible in the sense that they both write text, but that's it).