Originally Posted by Sawdustguy
Spice has been around forever. It is really used for electronic circuit CAD and simulation. I think it is not really geared to electric circuit analysis although I have not used it many years. I am much more familiar with Matlab and Simulink.
It actually is used for circuit analysis...it is similar to NASTRAN in that really it is an analytical solver that has had various versions of the same tool, and many front-ends put on it over the years by different companies and organizations.
If you look around, you can still get the original SPICE code(s) and write circuits into it using a text file. It does have the ability to analyze some solid-state devices, but if you are trying to simulate something like a fully integrated PID or similar device, it's unlikely you'll get that working very easily.
A quick google turns up this helpful Wikipedia page
with links at the bottom to both program source and tutorials.