Gnucap is a general purpose circuit simulator started by Albert Davis[1] in 1993[2]. The latest stable version is 0.35 from 2006. The latest development version (as of August 2010) is from December 2009 and is quite 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.
Gnucap+ is a proposed continuation, but as of June 2011 the site shows no activity or files.