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.

To Crowley’s credit, Closed Circuit is decidedly unflashy. But maybe that’s a liability: There’s a fine line between restrained and drab, and Closed Circuit falls just on the wrong side of it.