Useful Links
As a teacher, communicator and practitioner of math, physics and computer science I have found the following websites to be most helpful. Hover over each item to see a brief description.
General information

Wikipedia The encyclopedia I turn to first to learn about anything; to get dates, names, spelling, etc.

PhysOrg A news magazine for physics and other sciences.
Mathematics for students

Khan Academy Learn math by watching Salman Khan’s excellent videos.

PurpleMath Learn math by reading Elizabeth Stapel’s excellent explanations.
Mathematics for teachers

How math is used in technology Created by the faculty at my school, the BC Institute of Technology.

National Council of Math Teachers A national group that advocates for math education.

How to make your own Khan Academystyle videos The hardware you will need.

History of math and biographies of mathematicians Created by John O'Connor and Edmund Robertson of the University of St. Andrews.

Frequently asked questions about math Alex LopezOrtiz describes his creation as nontrivial mathematical trivia.
Physics and engineering for students

PhET physics simulations Absolutely outstanding! You can do science experiments on the computer screen.

Khan Academy physics lessons More videos by Salman Khan – these ones on physics.

All About Circuits  Electrical engineering Covers DC, AC, semiconductors and digital.
The web, HTML, CSS, JavaScript, forms, PHP

W3Schools An outstanding website! You see sample HTML, PHP, CSS and JavaScript code that you can modify and try out yourself.

HTML cheat sheet A summary of HTML tags, etc. Also available are cheat sheets on PHP, JavaScript and CSS.
 How to create a website

Chrome DevTools Tools for web authoring and debugging built right into Google Chrome.
 Search engine optimization
Programming

Stack Overflow A question and answer site for professional and enthusiast programmers.

Visual Basic Developers Resource Centre Get sample VB code and help from other programmers.

Inno Setup Create setup programs that install apps.
HTMLHelp documents

HTML Help Workshop download This tool from Microsoft is used to author .chm helpfiles

HTML Workshop tutorials Learn how the Algebra Coach jumps to specific topics in the ebook.
Other software

Maple One of the original symbolic math programs, developed at U. of Waterloo.

Mathematica Another one, begun by Stephen Wolfram, to do particle physics calculations.

MathType I use this app to typeset equations in my books.