Spice is a simulation program originally developed by Cal Berkeley. It has been commercialized by various vendors such a PSpice, HSpice, Analog Workbench, LTSpice, etc.
Version 2G6 was the last version written in Fortran and dates back to 1983, thus it's unlikely you can find a copy of that on the web. Why do you want that particular version? Do you have a computer that still runs Fortran?