This is a link to the author's full index of applets on the topic of electrostatics.  Topics include electric fields and forces, field lines, and equipotential lines.

In this closely related Java physlet, a test charge is displayed in a simulated parallel plate capacitor.  The charge shows a vector with the net force acting on it.  Students can move the charge around to view the direction of the force relative to the electric field and the equipotential lines.