GNU TeXmacs is a free scientific text editor, which was both inspired by TeX and GNU Emacs. The editor allows you to write structured documents via a wysiwyg (what-you-see-is-what-you-get) and user friendly interface. New styles may be created by the user. The program implements high-quality typesetting algorithms and TeX fonts, which help you to produce professionally looking documents. 
TeXmacs is also used as an interface for computer algebra systems.
It uses Guile/Scheme as extension language.
One of the fundamental ideas behind TeXmacs is that we wanted a WYSIWYG editor for structured documents with the same customizability as Emacs. Just like the native format of an Emacs document is basically a string, the native format of a TeXmacs document is a tree. This is particularly powerful in relation to Lisp-like extension languages (we chose Guile Scheme, because this is a standard extension language for the GNU Project and Gnome), which are particularly suited for tree manipulations.
There are exist many plugins for TeXmacs, among others for Maxima computer algebra system and R software environment for statistical computing.
There used to be also a TeXmacs wiki where you could find more help, but now it is only available in the Internet Archive.