Spice is a circuit simulator, more information about spice can be found at the
Professor A. Richard Newton gives a history of Spice as part of his
Presentation of the 1995 Phil Kaufman Award to Professor Donald O. Pederson
May, 2002 Berkeley Lab Notes article, "1972: The release of SPICE, still the industry standard tool for integrated circuit design"
comp.lsi.cad FAQ at
http://www.ece.ucdavis.edu/sscrl/clcfaq/faq/faq-N-20.html#N-20 has a question and some answers about Spice
See also the SPICE wiki page.
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5Spice.com Free for Non-commercial use under Windows
Spice for the Mac
http://ngspice.sourceforge.net/ is "an open source GPLed mixed-mode/mixed-level circuit simulator."