If you don't have a Java-enabled browser (for example, Microsoft Internet Explorer 5.0 or later) you probably can't play here. Most of these simulations are Java applets, requiring your browser to load about 1 megabyte of Java libraries to run the model.
If you are interested in modeling, but aren't a Java programmer, one of the most accessible and best-documented modeling platforms is NetLogo, a programmable modeling environment for simulating natural and social phenomena, available from Evanston's Center for Connected Learning. It is written in Java, but model procedures are written in Logo, the Lisp dialect developed by Seymour Papert around 1980 to teach children how to program computers. Logo is easy to learn and fairly capable.
NetLogo simulations of emergent behavior
Flocking, on the Northwestern University site, is modeled after Craig Reynolds' classic "Boids." Shows how flocking behavior can arise without leaders or pacesetters. There are numerous other models available from this site.
Hillclimb provides a simple experience of monoculture and diversity, cooperation and competition, and simple navigation -- comparing where you are to where you want to go.
Maze evokes human decision making. Generates a variety of mazes, and you can experiment with six different strategies for exploring them: random, wall-follow, past experience, expert opinion, monitor as you go, and flood-fill.
GRAZE # 1 was the first NetLogo model on Managing Wholes. It is a simulation of grazing a pasture. You control stocking rate, grazing and rest periods, and the rate of grass growth.
Segregation on the Northwestern University site shows how quickly groups form based on preferences.
Gecko is a more advanced modeling environment based on Swarm, which came out of the Sante Fe Institute. You can try several models here at Yale's Center for Biological Computing, such as "GrassWorld," a fun simulation of grass, grasshoppers, and spiders.
Modeling activities as antidote for the Centralized Mindset
"The detached stance encouraged by traditional science education tends to limit students' engagement with scientific ideas, making it difficult for them to build on their experiences and make strong personal connections to mathematics and science. There is a new appreciation for active participation, and not just distanced reflection, in the learning process." ---Mitch Resnick and Uri Wilensky, developers of StarLogo and NetLogo, respectively.