Ngspice is a mixed-level/mixed-signal circuit simulator. Its code is based
on three open source software packages: Spice3f5, Cider1b1 and Xspice. It is
the open source successor of these venerable packages. Many, many
modifications, bug fixes and improvements have been added to the code,
yielding a stable and reliable simulator.

WWW: http://ngspice.sourceforge.net/