LaTeX

 LaTeX

LaTeX is not proprietary, but a free, open-source software distributed under the LaTeX Project Public License (LPPL). 

It is available to use for free, with no license fees, and can be supported by donations. 

Origin

TeX, the original typesetting system, was created by Donald Knuth and released under an open-source license. 

LaTeX, a macro package built on TeX, was later developed by Leslie Lamport and is also free software.

Licensing 

The LPPL is a free software license that is compatible with the Debian Free Software Guidelines (DFSG). 

While the LPPL is not compatible with the GNU General Public License (GPL), it requires modified files to be clearly distinguishable from the originals to prevent "dependency hell".

Availability

LaTeX is available for most operating systems and is included in many TeX distributions. You can download and use it without paying for a license.
    

Support 

The developers invite users who are satisfied with LaTeX to support its maintenance and development through donations to the TeX Users Group