Lambda the Ultimate

In one sense, this is light enough that I feel bad posting it to LtU. In another, though, it's entirely relevant: this guy started out inventing a DSL for pen-strokes, combinations of them, and relations between these combinations, and ended up with a simple OCR-based 4-function calculator. This was one of the cleverest bits of just-because programming language work and contrived abstraction that I've seen in a long time. Sample:

   Zero is
          a large circle

     One is
          either
               a long vertical line
          or
               a long vertical line
               a very small slash line
               top of first joins top of second