ngspice is the open source spice simulator for electric and electronic circuits.
ngspice offers a wealth of device models for active, passive, analog, and digital elements. Model parameters are provided by collections, by semiconductor device manufacturers, or semiconductor foundries. The user adds circuits as a netlist and the output is one or more graphs of currents, voltages and other electrical quantities or is saved in a data file.
ngspice does not provide schematic entry. Its input is command line or file based. However, there are third party interfaces available.
ngspice is SPICE compatible. PSPICE or LTSPICE device model parameters and netlists for simulating discrete circuits are compatible. ngspice will also read HSPICE device libraries from semiconductor foundry PDKs for simulating integrated circuits.
ngspice builds on many operating systems. All popular Linux distributions, Cygwin or FreeBSD offer ngspice as well, as does Homebrew for macOS.
All things ngspice can be found here. The following specific links are available: F.A.Q., documentation, detailed manual, Mailing lists, discussion forums, and bug tracker.
ngspice can be downloaded from GitHub here and SourceForge here.
NGSPICE - SPICE SIMULATOR
Community
Ngspice Developer Community
Analog & M/S EDA
Free