Use Ghostscript, and build the devices under gs pbmraw, pbm.dev, etc. (See devs.mak under the sources for gs) The following converts file.ps in file.ppm:
gs -sDEVICE=ppmraw -sOutputFile=file.ppm file.ps
Or try using the Ghostscript tools ps2image.ps or pstoppm.ps. Or use the NETPBM tool pstopnm. Netpbm (a descendant of the PBMPLUS utilities) can then convert that image into whatever format you want.
You can also use the ImageMagick-tools program ``convert''. Try ftp.x.org in contrib/ImageMagick.tar.Z.
If you have the new xv installed, it can now read PostScript files, and it stores them to any format.