This java applet is a magnetostatics demonstration which displays the magnetic 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 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.
