ngspice

ngspice was developed as an accessible and Open Source mixed-level / -signal circuit simulation app and hence, can come in handy for anyone who wants to build or experiment with electric and electronic circuits.

The program's design is based on Spice3f5, Cider1b1 and Xspice. ngspice is a project that was specially developed to improve the spice3f5 circuit simulator. According to the developer, the app is also compatible with PSPICE and LTSPICE, but can also read HSPICE device libraries from semiconductor foundry PDKs for simulating integrated circuits.

Generally speaking, circuits can include diodes, transmission lines, JFETs, MOS and bipolar transistors and other devices that are all interconnected. The application can be used to simulate both standard and digital circuits, regardless of the level of complexity, from single gates to complex gates. You have dozens of models available that can be used in analog, passive, active and digital elements.

You can get started by adding the circuits as a netlist and an output, which can have one or more graphs of current, voltage and other electrical quantities. Then again, you should bear in mind that the app does not provide schematic entry and the input is available by either file or command line.