The original circuit simulation software, called Simulation Program with Integrated Circuit Emphasis, or SPICE as it is more commonly known, was originally developed at the University of California Berkeley in the 1970s under an open source license. This is the reason for the major versions of SPICE available now decades after the original was released, not all of which are as open or free as we would like. LTspice is a popular choice, however, as far as a truly free and open source circuit simulator goes you might want to try QucsStudio.
Qucs, or Quite Universal Circuit Simulator, is the foundation of the circuit simulation software, with QucsStudio providing the GUI layer for ease of use. QucsStudio supports a wide variety of circuit components and models much in the same way as other more popular SPICE programs, including semiconductor devices, passive components, and digital logic gates. Qucs also uses SPICE-based simulation, which can model various types of circuit behavior, such as DC, AC, transient and small-signal analysis.
Unfortunately, only Windows versions are available, and while some may have some success running it under WINE, we hope that the fact that it’s released under such a permissive license leads to a Linux version at some point . However, there are plenty of other options for those of us running non-Windows operating systems. Here is a review of 30 of them.
Thanks to (Electroagenda) for the tip!