In response to Barney's request about information on software tools, here is a little more information:
DYNAMO, developed at MIT's SD Group especially aimed at non-programmers Windows environment
contact: Pugh-Roberts in Cambridge, MA
PowerSim, developed in Norway award winning software for creating SD models and simulations Windows environment
contact: ModellDate in Bergen, NORWAY
STELLA, developed by Richmond, Peterson, & Vescuso Macintosh & now PC--model construction/simulation tool being used in Germany for teaching elementary physics
Vensim, developed by Ventana Systems industrial strength with many excellent features, including reality checks and statistical tools
contact: Ventana Systems
These, and other tools are discussed in Morecroft & Sterman (Eds.) Modeling for Learning Organizations (1994). All of these tools offer strong graphical support for representing models. In addition, other graphical tools exist which can be used in the knowledge elicitation/representation process to augment these tools.
Here are some interesting web sites to visit with still more information:
http://www.ncsa.uiuc.edu/Pubs/access/92.4/92.4Stella.html
http://www.radix.net/~crbnblu/indivs.html
http://sysdyn.mit.edu/sdep.html (which confirms many of Steve's points)