GnuCap is a general purpose circuit simulator. GnuCap was formerly known as ACS. GnuCap performs nonlinear dc and transient analyses, 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. Spice compatible models for the MOSFET (level 1-7) and diode are included in this release. Since it is fully interactive, it is possible to make changes and re-simulate quickly. The interactive design makes it well suited to the typical iterative design process used it optimizing a circuit design. Unlike Spice, the engine is designed to do true mixed-mode simulation. Most of the code is in place for future support of event driven analog simulation, and true multi-rate simulation. If you are tired of Spice and want a second opinion, you want to play with the circuit and want a simulator that is interactive, you want to study the source code and want something easier to follow than Spice, or you are a researcher working on modeling and want automated model generation tools to make your job easier, try GnuCap.