Qucs-S is a spin-off of the Qucs cross-platform circuit simulator.
"S" letter indicates SPICE.

Qucs-S project uses free SPICE circuit simulation kernels to sumulate
circuits. It merges the power of SPICE and the simplicity of the Qucs
GUI.

Backends:
* Ngspice is the default backend recommended for use in Qucs-S.
  Ngspice is powerful mixed-level/mixed-signal circuit simulator. Most
  of industrial SPICE models are compatible with Ngspice. It has an
  excellent performance for time-domain simulation of switching
  circuits and powerful postprocessor.
* XYCE is a new SPICE-compatible circuit simulator written by Sandia
  from scratch. It supports basic SPICE simulation types and has
  advanced RF simulation features such as Harmonic balance simulation.
* SpiceOpus is developed by the Faculty of Electrical Engineering of
  the Ljubljana University. It based on the SPICE-3f5 code.
* Qucsator as backward compatible.