Qucs is a GPL circuit simulator. And if you want the GUI option, you might want to try out QucsStudio, which uses Qucs under the hood, and is free to use, but binary-only. (Editor’s note ...

We are big fans of the Falstad circuit simulator. Sure, it isn’t perfect, but there’s nothing else like it when you want to whip up a simple circuit. But we were blown away when we saw a more ...