PreProcessors

Standard preprocessors:

cw - Prepares constant-width text for the troff command; old UNIX System III ?? http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.doc/cmds/aixcmds1/cw.htm; not anymore necessary? I found no c-code
eqn - equation formatter (http://www.zen89632.zen.co.uk/Groff/Eqn/eqnguide.pdf)
grn (Gremlin) - another drawing formatter
pic - drawing formatter
preconv - convert encoding of input files (e.g. UTF-8) to something groff understands
refer - bibliographic references
soelim - resolve all .so requests
tbl - table formatter

Optional preprocessors:

aspic - Aspic's concept is similar to the "pic" command. Output is either encapsulated PostScript?, or Scalable Vector Graphics (SVG). ftp://ftp.csx.cam.ac.uk/pub/software/wordprocessing/unix/aspic/aspic-1.05.tar.gz
chem - available at Brian Kernighan's old webpage http://cm.bell-labs.com/who/bwk/. For typesetting chemical diagrams.
dformat - a program for typesetting data formats; described in http://cm.bell-labs.com/cm/cs/cstr/142.ps.gz; source at http://www.troff.org/dformat. Because of internationalization issues it is useful to call it "LANG=C dformat"
flo - flow charts, could not find the source; Wolfman, T. and Berry, D.M. ``flo--A Language for Typesetting Flowcharts, User's Manual, Faculty of Computer Science, Technion, Haifa, Israel, 1990; http://se.uwaterloo.ca/~dberry/FTP_SITE/tech.reports/flo.doc.pdf
grap - http://www.lunabase.org/~faber/Vault/software/grap/ - Grap is a language for typesetting graphs in groff. It is an expressive language for describing graphs and incorporating them in typeset documents. It is implemented as a preprocessor to the pic language. The original grap was made by Brian Kernighan and Jon Bentley and documented in Bell Labs CSTR 114: http://cm.bell-labs.com/cm/cs/cstr/114.ps.gz

Other preprocessors, unknown license status: