TeX
TeX is a typesetting system that was designed for creating high-quality documents, particularly those that contain mathematical and scientific equations. Its main uses include typesetting books, articles, and academic papers, as well as producing professional-looking documents with complex formatting requirements.
Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.
Repositories on GitHub: 280,431