Davey,
I use Performance Trends suspension analyzer.
http://www.performancetrends.com/SuspAnzr.htm
You can download a demo which gets you the basic program.
This is a very powerful program but it is not very user friendly and you need to know what your design and performance goals are.
I had a lot of help from another user on the Pro-Touring forum when I did the front suspension design for my Astro. It is kick-ass though and should perform very well if I every get it built....
Mark