Hi Chief Delphi,
I have been working on a simple system simulator and I think it's at a state where someone else might find it useful now.
It's a bit bare bones at the moment but I've found that having functions that can take the integral over time or plot graphs have made my actual simulation code much shorter and more understandable.
In this example, I've implemented a flywheel system that's being controlled by a PID loop with feed forward.
A live demonstration can be found here:
http://tigerhuang.com/simulation
The source code can be found on github:
http://github.com/tigerh/simulation