3-D Magnetostatic Fields 1.3c
Physics simulation that demonstrates magnetic fields in various 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 little magnets). This helps you visualize the field. You can also view the vector potential (A).
The menu in the upper right has a variety of different fields to choose from.