has a demo version of OrCAD Capture, OrCAD Capture CIS Option, PSpice® A/D, PSpice AA, OrCAD PCB Editor and SPECCTRA® for OrCAD.
There are limitations in the size and complexity of the design, but it's free.
The bare SPICE simulator is free, and has no limitations of complexity, but it has a 70s era user interface (i.e. none), and you have to create a file simulating a stack of punch cards with reference designators, values, node numbers, simulation parameters, et cetera. It's very painful to use, but this is the engine that many other programs use, dressed up in a nice schematic capture user interface.
There are probably several shareware applications you can use with this, depending on your operating system.
Have fun, but you might want to search these forums for discussions concerning how useful these simulators are in various applications.