There are indeed a variety of software that writes organic chemistry formulas. I will describe two types of software to be more confident that I am addressing what is meant in the question.
First there is software that allows one to create structural formulas of really any chemical formula, organic or inorganic, such that you create a "stick figure" kind of structure, or "ball and stick" where the spheres represent different elements and the sticks represent the bonds. The same software also provides "space-filing" structures of molecules that can be rotated in all three dimensions. This is very helpful in understanding isomerism in particular. Many commercial software packages could be found online - but I will offer one that can be downloaded and used for free: Avogadro ... https://avogadro.cc/ This is a relatively easy software program to use, and it's free!
A second class of free software is one for authors: books, journals, posters, and presentations that require professional quality typesetting. For this purpose I highly recommend the use of the LaTeX typesetting system in conjunction with extensions particularly suited for writing chemistry. The extensions include mhchem, chemformula, and especially chemfig - which is the preferred package for writing organic molecules and reactions. This software will require a bit of a learning curve to use productively, but the effort is well worth it.
For reference see:
https://www.latex-project.org/
ftp://www.ctan.org/tex-archive/macros/latex/contrib/mhchem/mhchem.pdf
http://mirrors.ibiblio.org/CTAN/macros/latex/contrib/chemformula/chemformula_en.pdf
http://tug.ctan.org/macros/generic/chemfig/chemfig-en.pdf
