The most popular extension is probably LaTeX. Emacs supports TeX, LaTeX and Texinfo files with their own major modes,
‘texinfo-mode’. Actually, latex-mode is very similar to tex-mode. See also AUCTeX, a popular (La)TeX editing mode / environment for emacs.
TeXmacs is a more conventional WYSIWYG word processor that is inspired by Emacs with regard to its keybindings, and by LaTeX with regard to math support. It uses TeX fonts and typesetting algorithms for its operation, but is neither actually using either Emacs or TeX/LaTeX. It has its own document format, from which it can export and import with varying success to LaTeX and HTML.