GNU m4 is a macro processor often used on Unix systems. For example, m4 is used for generating sendmail configuration files and by the GNU Autoconf system.
I wrote the first version of GNU m4 back in the 1980s, but maintenance and further development has since passed on to others.
GNU m4 has a web site with online manual at http://www.gnu.org/software/m4/.
GNU m4 development information and CVS repository is at http://savannah.gnu.org/projects/m4/.
My old, and now out-dated, mini-site on GNU m4 is here.