Where Can I Find a Free Electromagnetic Field Simulator?

[Astrum]

Is there any program (preferably free) that simulates electromagnetic fields and their interactions with charged particles?

I've found a couple gravity simulators, but none for EM, which isn't any fun, because I reckon it would be great to play with a realistic EM program.

 

[Drakkith]

Try looking up particle-in-cell programs.