The Qucs is one of the featured projects for the week (June 1, 2015), which appear on the front page of SourceForge.net:
Qucs is a circuit simulator with a graphical user interface. The software aims to support all kinds of circuit simulation types such as, e.g. DC, AC, S-parameter, Transient, Noise, and Harmonic Balance analysis. Pure digital simulations are also supported.
[ Download Quite Universal Circuit Simulator ]