My favourite is Texas Instruments PC-Scheme. Complete with Emacs-like editor. You could compile and evaluate regions in the editor. It is amazing what you can do in 2MB or even 640K.
There's also s4iof (R4RS), s5iof (R5RS) and skint (R7RS, but hey 5 .c files), all from the same people. I might interject that R4RS might seem preferable for the minimalist cadre.