A man I admire, Richard Feynman.
A pretty cool WYSIWYM (What you see is what you mean) typesetting system, LaTeX. Especially useful if you want to prepare reports, papers and so on. The LaTeX stack exchange is a good place for useful stuff about LaTeX.
LyX is another software based on LaTeX. It is a useful WYSIWYG (what you see is what you get) document preparation system and handy if you want to prepare professional presentations or reports but do not want to delve into the intricacies of LaTeX.
The open directory project, a useful site to search for anything technical and non technical.
An essay titled “The unreasonable effectiveness of mathematics” by Richard Hamming.
Free ebooks (legal)