More on System Dynamics for Cheapskates
If you're dealing with random data, there are also R and ViSta.
Just as I was about to post this, Gene Shackman sent out a pointer to a list he has created of statistics packages.
Bill Harris founded Facilitated Systems in 1999 to help people by helping the organizations in which they spend so much of their time. He uses a number of approaches to help them make sense of the puzzles and problems organizations face.