Qucs

Qucs stands for Quite Universal Circuit Simulator. So far Qucs is not yet finished, but it is already packed with features. Take a look at the screenshots to get a feel for what it can do.

Qucs is an integrated circuit simulator which means you are able to setup a circuit with a graphical user interface (GUI) and simulate the large-signal, small-signal and noise behaviour of the circuit. After that simulation has finished you can view the simulation results on a presentation page or window.

Qucs, briefly for Quite Universal Circuit Simulator, is a circuit simulator with graphical user interface (GUI). The GUI is based on Qt® by Digia®. The software aims to support all kinds of circuit simulation types, e.g. DC, AC, S-parameter, Harmonic Balance analysis, noise analysis, etc.

Qucsator, the simulation backend, is a command line circuit simulator. It takes a network list in a certain format as input and outputs a Qucs dataset. It has been programmed for usage in the Qucs project but may also be used by other applications.
The Qucs GUI is well advanced and allows setting up schematics and presenting simulation results in various types of diagrams. DC, AC, S-parameter, noise and transient analysis is possible, mathematical equations and use of a subcircuit hierarchy (with parameterised subcircuits) are available. Qucs can also import existing SPICE models for use in your simulations.

While Qucs is well developed, we still strive to improve the project, and will always welcome offers of help. Qucs doesn't just need help with programming, it also needs graphics, high quality example schematics, tutorials for the website and lots more.

Qucs comes with an array of components and models. Third party models including HICUM, BSIM2, BSIM3 and BSIM6 can be compiled and loaded into the simulator. It also provides many semiconductor based components and models such as OpAmps, Diodes, MOSFETs, PMOSFETs and many more. Some example schematics can be found here which demonstrate some of the abilities of Qucs, and many more examples are provided with the program.
Operating Systems
Qucs is currently developed under the GNU/Linux OS using the standard autotools with no special effort to support other operating systems. However Qucs is said to be successfully compiled and run on Windows, Solaris, NetBSD, FreeBSD, macOS, Cygwin... help is needed!
License
Qucs is released under the GPL license and so it is free for free programmers and users!