Looking for simulation software for non-majors course

[Steve Taub]

We are planning a lab in our non-majors' bio course to show use of
computer simulations in biosciences. Software should deal with subject
of interest to lay folk--i.e., clinical, environmental, etc. Also 
should be inexpensive (public domain, if possible.) and, if possible,
interactive, but not resembling a computer game. Any suggestions? 
Sources? Thanks very much for any help. Steve Taub, Biology, George
Mason University.