Qt Universal Circuit Simulator is a circuit simulator with graphical user interface. The software aims to support all kinds of circuit simulation types, e.g. DC, AC, S parameter, harmonic balance analysis, etc. It comes with Qucsator which 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 can also be used by other applications.
For people who are familiar which such simulators they should remember ADS from Agilent Technologies, Microwave Office from AWR and all the other simulators.