This toolbox contains a main GUI that allows you to design a circuit and simulate it, very similar to Pspice. You can use nonlinear devices such as diodes and bipolar transistors, or why not define your own component model.
If you feel more comfortable with netlist, there are routines for calculating the resulting node voltages and currents from netlists. Netlists and schematics are entered in matrix format, where each row corresponds to a schematic item or a component.
There is a model library tool that enables you (and the simulator) to keep track on your models. All models are assigned a unique pointer/ID which is used for the schematic and netlist matrices. The model library does not contain information about inherent "models" such as resistors, capacitors or ideal op-amps.
The simulator make use of the MNA formulation of a circuit. See  Eric Cheever, http://www.swarthmore.edu/NatSci/echeeve1/Ref/mna
The tar.gz version is available on my homepage:
This version is very beta since it took about one and a half month to develop. So don't expect it to be perfect. :-)
(See the accompanying readme-file about important notes and planned updates.)
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