The best free alternative to OrCAD (All Products) is KiCad, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to OrCAD (All Products) and many of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to OrCAD (All Products) are QUCS (Free, Open Source), Autodesk EAGLE (Free Personal), LibrePCB (Free, Open Source) and LTspice (Free).
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.
The gEDA project has produced and continues working on a full GPL'd suite and toolkit of Electronic Design Automation tools. These tools are used for electrical circuit design, schematic capture, simulation, prototyping, and production.
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.