This is the companion applet, "Magnetic Field Demonstration", in which users can add currents coming into or out of a simulated grid and see the fields created.  Included is a selection of pre-created fields, including bar magnets, loops, opposing magnets, and coils.

This is the full index of interactive Java simulations by the same author on Electricity and Magnetism.  This particular collection is devoted to topics typically covered in a second semester introductory physics class.