SIOD = Scheme in One Defun (also rumored to mean "Scheme in One Day"...)
SIOD is a "small-footprint" Scheme implementation, written by George J. Carrette.
It's rather limited as a Scheme implementation, but nevertheless it's used as the Scheme interpreter of some libre programs. Examples include Festival speech synthesis program, SIAG spreadsheet, and GIMP's Script-Fu extension.