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/