The Open Office set (Writer, Draw, Calc etc.) can get you started with WSYG pages quite nicely. Tex & Latex were traditionally used to mark up math notation and other txt ligature styles for printing and are well documented. There are also a number of specialzed diagramatic tools for scientific venn-euler formats like Mattlab and Dia.
GIMP is the mainstay swiss army knife of image compositors. However it comes with a high learning curve and the ability to assemble massive documents and transmogrify them into all sorts of formats.