Text Processing Tools

A2ps
ASCII to PostScript converter with pretty-printing.
Ghostscript/Ghostview/Gv
PostScript/PDF viewer, interpreter and conversion
Groff
GNU replacements for nroff, troff, eqn, tbl, soelim, ...
SGML Tools
Format SGML documents and output HTML, LaTeX, Texinfo,  PostScript, DVI, RTF and plain text.
TeX/Texinfo
Document formatting and typesetting system 
Enscript
Extensible text to PostScript converter
Par
Sophisticated paragraph reformatter