auctex – Emacs support files for TeX

AUC TeX is an extensible package that supports writing and formatting TeX files for most variants of GNU Emacs. In recent versions of XEmacs, AUC TeX is usually already available as a package, though not necessarily up to date. AUC TeX supports many different TeX macro packages, including AMS-TeX, LaTeX, and Texinfo. ConTeXt is not yet supported, volunteers to change this would be welcome.

Note that AUC TeX bundles its own copy of the LaTeX preview package.