Introduction to Cryptography : A short introduction to traditional pen and paper cryptography that I wrote many years ago.
Learn LaTeX : LaTeX is a text typesetting system that beats pretty much anything else a normal person can get. This is a PDF file of an introduction to LaTeX that I wrote long ago. Luckily, LaTeX is pretty well done and doesn't change that much. More than once, I've found a template or snippet from many, many years ago on the web and used it without any problem. So, this is still probably a useful introduction for anyone who wants to use LaTeX.
Learn LaTeX : PS (PostScript) file of above. Useful if you want to do misc things that are not supported by PDF to the file.
Learn LaTeX : The original LaTeX source file. Because it is LaTeX, it is just plain text, with markup.