Online Circuit Simulator
This is a basic Java applet that can run in a browser and can simulate electronic circuits, displaying voltage and current reading for various components as selected. While very simple to use, it is quite useful in describing basic circuits and displaying the effects of different components on a circuit. It also has a number of common circuits already designed which can be edited.
This simulator is free to use online. There is no downloadable version.
Basic directions are available at http://www.falstad.com/circuit/directions.html.