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The most popular FOSS spice simulator is NGSpice: http://ngspice.sourceforge.net/

None of these tools have changed much since the 90's - just like the EDA industry in general

Also see Qucs-S, which is a GUI for ngspice and a few other engines.

Oo that looks nice! Qucs had a surprisingly great GUI - you wouldn't believe it was an EDA tool!