If you have a Furby and would like to replace its brain with a custom 8051-based controller, visit the Furby Upgrade Center. The board runs HurbOS v0.1 and allows you to program your Furby with custom behaviours by writing C or Assembler code. You can also download new sounds file in WAV format. Skinning your Furby and installing an 8051 board will void the warranty but it sounds like fun.