Risp?
Mike Fogus implemented Lisp in 40 lines of Ruby code, then went back and tweaked it to be 32 lines. And desipte Arild's comment in the audio, its not like an 'obfuscated C' contest either.
Take 5 minutes, peruse the code, and stretch your mind a bit. You might just learn a thing or two about Ruby while you're at it!