A basic circuit simulator which is does an amazing job showing the movements of current and how that affects various elements of a circuit. With the ability to trend components of the circuit this simulator can be very helpful in a pinch.

CircuitLab is an interactive circuit simulator which has the capability to simulate DC, AC, and Logic circuits. CircuitLab also has a free interactive textbook for those interested in learning more about electronic circuits. The motto of the book is "One Step Deeper", with the idea being to connect the mathematics, physics, and practical application of electronic circuits.

Ultimate Electronics Interactive Textbook

W3 Schools online code editor for HTML and Javascript.

A free and simple website which will take an uploaded image and give you mock up design of how that image would look on a mobile device!