The following is a collection of guides, hacks, HOWTOs, tips and tricks – I have acquired over the years – for various programs and concepts. I will fill this list over a period of time, so it's far from complete – and probably never will be.
Mathematics as for many hackers, is an interesting and fascinating topic. It's literally a language to explain everything.
geogebra is an interactive geometry, algebra, statistics and calculus application, intended for learning and teaching mathematics and science from primary school to university level.
groff (also called GNU troff) is a typesetting system that creates formatted output when given plain text mixed with formatting commands. Output may be PostScript or PDF, html, or ASCII/UTF8 for display at the terminal.