Quite Universal Circuit Simulator

Quite Universal Circuit Simulator (Qucs) is a free-software electronics circuit simulator software application released under GPL. It offers the ability to set up a circuit with a graphical user interface and simulate the large-signal, small-signal and noise behaviour of the circuit. Originally, Qucs was composed of a circuit simulator "qucs-core", now Qucsator, and a GUI for schematic entry and plotting.

Source: Wikipedia — Quite Universal Circuit Simulator (CC BY-SA 4.0)