Xyce is described as '(zi¯s, rhymes with “spice”) is an open source, SPICE-compatible, high-performance analog circuit simulator, capable of solving extremely large circuit problems by supporting large-scale parallel computing platforms' and is a circuit simulator in the education & reference category. There are six alternatives to Xyce for Windows, Linux and Mac. The best alternative is QUCS, which is both free and Open Source. Other great apps like Xyce are Solve Elec, Ngspice, proteus VSM and Qucs-S.
The Quite Universal Circuit Simulator (QUCS) is an integrated circuit simulator allowing the simulation and graphical presentation of results of large-signal, small-signal and noise behavior in circuits.
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.