Ngspice is a mixed-level/mixed-signal circuit
simulator based on three open source software
packages: Spice3f5, Cider1b1, and Xspice. Spice3
is the most famous and widely used circuit
simulator. Cider is mixed-level simulator that
includes Spice3f5 and adds DSIM, a device
simulator. Cider couples the circuit level
simulator to the device simulator to provide
greater simulation accuracy (at the expense of
greater simulation time). Xspice is an extension
to Spice3 that provides code modeling support and
simulation of digital components through an
embedded event driven algorithm.