Computing Environments Needed For The Models
Mathematica
Mathematica is a symbolic algebra environment available from Wolfram Research. The toolkit is actively developed with Mathematica v5.2, but it's fairly vanilla code that should work as far back as v4.0 or earlier. People who don't have access to a full version of Mathematica can still view the models and selected results using the free program MathReader.
Matlab
Matlab is a numerical maths environment available from The Mathworks. As well as the base program, you will almost certainly also want Simulink and the Control Systems Toolbox. The Matlab wrapper programs for exported results are a mixed bag. The Matlab code should work back to Matlab R6, but some of the Simulink diagrams require R7.
E2E
E2E is a modeling package developed in-house at LIGO to study the opto-mechanical performance of suspended interferometers. It has a Simulink-like graphical front-end called Alfi.