netpbm-mar1994 was a subsequent distribution of the same free software augmented by contributed filters and formats. netpbm-1mar1994-free.tar.gz (from a Red Hat source RPM) is a subset of this distribution where the only contribution with an unworkable license (hpcdtoppm.c) was removed.
ivtools drawing editors support the import of all six pbmplus formats (pgm/pgm/ppm in ASCII and binary). The pathname of these image files can be entered directly into the string editor of the import dialog box, or if the "from command" checkbox is set, an arbitrary command line can be entered that generates a pbmplus image format.
A wider variety of raster formats is supported by use of the "anytopnm" script, of which a modified and improved copy is incorporated directly into ivtools.
Linux binaries for netbpm and pbmplus available from sunsite.unc.edu and other Linux distributions.
A revived version of the Netpbm repackaging of pbmplus can be found at http://sourceforge.net/projects/netpbm/