When I need to produce any document more complicated than simple notes, I use the LaTeX typesetting system. It allows me to write markup in a text editor to produce nicely typeset documents with equations in PDF format, rather than fiddling with a WYSIWYG editor like LibreOffice. My typical environment for writing LaTeX is gedit, a PDF viewer, and a terminal to compile the document.
Gummi is a LaTeX editor that integrates all the pieces I need for writing into one application. The left pane is an editor with syntax highlighting and spell checking. The right pane renders the current state of the document or shows errors as you type. There are graphical shortcuts for operations involving simple text formatting, images, tables, matrices, and bibliographies. The New from Template function means I won’t have to hunt down an old document to use as a template for a new one.
I was disappointed that the spell checker was not any smarter than the one in gedit about not marking LaTeX keywords as misspelled. It would also be nice if the preview could be scrolled continuously like a PDF in evince.
Ubuntu packages for Gummi are available from the Gummi PPA. Be aware that Gummi depends on LaTeX, which will be a several hundred megabyte download if you don’t have it already.
There’s no shortage of editors for LaTeX in the Ubuntu repositories. Another similar application is TeXworks, which has a better spell checker, but puts the preview in a separate window and seems to have broken syntax highlighting.