Nxhtml is a project mainly written in ..., it's free.
Git mirror of Emacs nxhtml project
To install nXhtml put this in your .emacs:
(load "YOUR-PATH-TO/nxhtml/autostart.el")
where autostart.el is the file in the same directory as this readme.txt file.
Note 1: If you are using Emacs+EmacsW32 then nXhtml is already installed.
Note 2: If you are using Emacs 22 then you need to install nXml separately. (It is included in Emacs 23.)
Note 3: You may optionally also byte compile nXhtml from the nXhtml menu (recommended).
Some files that were previously distributed with nXhtml are now in Emacs' development repository. Distributing them also with nXhtml is a bad idea since that can lead to that the wrong file is loaded. They are therefore not distributed with nXhtml anymore.
Instead you can (if you do not have the files in your Emacs) in many cases use the version from the repository. To do that you can currently start from
http://cvs.savannah.gnu.org/viewvc/emacs/emacs/lisp/
Files you can download and use this way are for example
js.el (JavaScript, formerly called espresso.el) htmlfontify.el
If you do that I suggest that you put these files in a special directory and add that to load-path in your .emacs and make that adding to load-path depend on your Emacs version so that they will not be loaded when you have upgraded your Emacs.
Note that if you want to use nxml-mode (and it is not in your Emacs) you should not download it from Emacs' development directory. Instead go to
http://www.thaiopensource.com/download/