GnuCap is the Gnu Circuit Analysis Package.

The primary component is a general purpose circuit simulator. It performs
nonlinear dc and transient analyses, fourier analysis, and ac analysis. It is
fully interactive and command driven. It can also be run in batch mode or as a
server. Spice compatible models for the MOSFET (level 1-7) and diode are
included in this release.

GnuCap is not based on Spice, but some of the models have been derived from
the Berkeley models.

Unlike Spice, the engine is designed to do true mixed-mode simulation. Most
of the code is in place for future support of event driven analog simulation,
and true multi-rate simulation.

If you are tired of Spice and want a second opinion, you want to play with the
circuit and want a simulator that is interactive, you want to study the source
code and want something easier to follow than Spice, or you are a researcher
working on modeling and want automated model generation tools to make your job
easier, try GnuCap.

WWW: http://www.gnucap.org/