This java applet is an electrostatics demonstration which displays the electric field in a number of situations. You can select from a number of fields and see how particles move in the field if it is treated as either a velocity field (where the particles move along the field lines) or an actual force field (where the particles move as if they were charged particles). This helps you visualize the field.
The menu in the upper right has a variety of different fields to choose from.
See also the 2-D version.
Huge thanks to Bob Hanson and his team for help converting this applet to javascript.
