LaTeX is used by most mathematical scientists, like myself, to typeset most of their writing. At a one-day workshop in economics yesterday for example, at the University of Sydney, most of the speakers had done their slides using LaTeX. The basis of LaTeX typesetting is a source file which consists of plain text which is run through the LaTeX engine to produce a PDF or other display file. This meands that one is usually working with at least two files at the same time - the source code and the rendered output. Now, the excellent text editor gedit has a plugin which displays the rendered PDF (or DVI) file right next to the source code, inside the editor!
This is a really nice feature and gedit's nice syntax highlighting and wealth of other plugins, makes it a pleasure to use. Gedit is, of course, the default text editor in Linux' GNOME desktop environment and will probably run in other operating systems as well. The LateX plugin can be found at live.gnome.org/Gedit/LaTeXPlugin.