The best public domain DVI to PostScript conversion program, which runs under many operating systems, is Tom Rokicki's dvips. dvips is written in C and ports easily. All current development is in the context of Karl Berry's kpathsea library, and sources are available from the TeX live repository.
An VMS versions is available as part of the CTAN distribution of TeX for VMS.
A precompiled version to work with emTeX is available on CTAN.
Macintosh users can use either the excellent drivers built into OzTeX or Textures, or a port of dvips in the CMacTeX package.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=dvips