docstrip program distributed with LaTeX does not typeset anything, it is used to extract source code (while evaluating boolean guards) from documented sources, and to optionally install them in specified locations.
Another such program distributed with LaTeX is
latex latexbug interacts with the user to create a draft email of a bug report with several fields filled in accurately with information such as the latex version being used, it is of course written in LaTeX.
makebst is an interactive program written in TeX that queries the user on the desired properties of the bibliography style and then produces a custom bibtex style file to meet the requested specification.
basix is an interpreter for the BASIC language written in TeX.
TeX can also of course solve sudoko.
One other one I forgot to mention xmltex is a namespace aware XML parser written in TeX. It can be used for typesetting documents, but also other things. Its default behaviour is not to typeset anything and just to echo the parse tree to the log file.