The original circuit simulation software, called the Simulation Program ... not all of which are as open or free as we might like. Qucs is a GPL circuit simulator. And if you want the GUI option ...

[FightCube] wrote in recommending a very helpful piece of circuit simulation software for those of you ... and best of all, it’s free. He has shared one basic circuit he built as a demonstration ...

Using EM simulation to accelerate and simplify your design workflow to create better designs.