Gnucap is described as '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' and is an app in the Education & Reference category. There are four alternatives to Gnucap for Mac, Windows and Linux. The best alternative is Ngspice, which is both free and Open Source. Other great apps like Gnucap are McCAD 3SPICE (Paid), MacSpice (Free) and PrimeSim HSPICE (Paid).
Ngspice is a mixed-level/mixed-signal circuit simulator. Its code is based on three open source software packages: Spice3f5, Cider1b1 and Xspice. Ngspice is part of gEDA project, a full GPL'd suite of Electronic Design Automation tools.
McCAD 3-SPICE is based on the latest version (3F5) of Berkeley's SPICE 3 and permits interactive mixed mode circuit simulation. It was developed specifically to take advantage of the processing speeds available thus permitting extremely fast simulations.
As IC geometries continue to shrink, the need for an accurate circuit simulator is critical. Designers require a highly accurate circuit simulator to precisely predict the timing, power consumption, functionality, and yield of their designs.