The AMS publishes a “Short Math Guide for LaTeX”, which is available (in several formats) via http://www.ams.org/tex/short-math-guide.html
Herbert Voss is developing a parallel document, that is also very useful; it’s part of his “tips and tricks” and a copy is maintained on CTAN.
Peter Smith’s “LaTeX for Logicians” covers a rather smaller subject area, but is similarly comprehensive (mostly by links to documents on relevant topics, rather than as a monolithic document.
Keith Reckdahl’s “Using Imported Graphics in LaTeX2e” is an excellent introduction to graphics use, though it’s slightly dated in not discussing anything other than the dvips route. Available on CTAN, but again without sources.
Vincent Zoonekynd provides a set of excellent (and graphic) tutorials on the programming of title page styles, chapter heading styles and section heading styles. In each file, there is a selection of graphics representing an output style, and for each style, the code that produces it is shown.
An invaluable step-by-step setup guide for establishing a “work flow” through your (La)TeX system, so that output appears at the correct size and position on standard-sized paper, and that the print quality is satisfactory, is Mike Shell’s testflow. The tutorial consists of a large plain text document, and there is a supporting LaTeX file together with correct output, both in PostScript and PDF, for each of A4 and “letter” paper sizes. The complete kit is available on CTAN (distributed with the author’s macros for papers submitted for IEEE publications).
Documentation of Japanese TeX use appears at least twice on the web: Haruhiko Okumura’s page on typesetting Japanese with Omega (the parent page is in Japanese, so out of the scope of this FAQ).
One “Tim” documents pTeX (a TeX system widely used in Japan) in his “English notes on pTeX”.
Some university departments make their local documentation available on the web. Most straightforwardly, there’s the simple translation of existing documentation into HTML, for example the INFO documentation of the (La)TeX installation, of which a sample is the LaTeX documentation available at http://www.tac.dk/cgi-bin/info2www?(latex)
More ambitiously, some university departments have enthusiastic documenters who make public record of their (La)TeX support. For example, Tim Love (of Cambridge University Engineering Department) maintains his deparment’s pages at http://www-h.eng.cam.ac.uk/help/tpl/textprocessing/
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=tutbitslatex