Gnucap is a general purpose circuit simulator started by Albert Davis in 1993. It is part of the GNU Project. The latest stable version is 0.35 from 2006. The latest development version (as of June 2014) is from April 2013 and is usable.
It performs nonlinear DC and transient analysis, Fourier analysis, and AC analysis linearized at an operating point. It is fully interactive and command driven. It can also be run in batch mode or as a server. The output is produced as it simulates.
References
GNU Circuit Analysis Package Wikipedia(Text) CC BY-SA