An old tool used to format Unix (and Tcl/Tk) man pages. Alas, it is one of the less well-known formats about despite its widespread use even now. The sister tool troff produces rather pretty output, similar in quality to LaTeX. groff is the most common modern implementation, and can produce fine nroff-level output even to terminals, so long as they are UNICODE-enabled and are using a suitable font.
The basic format is documented in CSTR#54, which is downloadable (as gzipped PostScript) from http://cm.bell-labs.com/cm/cs/cstr.html