MASON is a fast discrete-event multiagent simulation library core in Java, designed to be the foundation for large custom-purpose Java simulations, and also to provide more than enough functionality for many lightweight simulation needs. MASON contains both a model library and an optional suite of visualization tools in 2D and 3D.
MASON is a joint effort between George Mason University's Evolutionary Computation Laboratory and the GMU Center for Social Complexity, and was designed by Sean Luke, Gabriel Catalin Balan, Keith Sullivan, and Liviu Panait, with help from Claudio Cioffi-Revilla, Sean Paus, Keith Sullivan, Daniel Kuebrich, Joey Harrison, and Ankur Desai.
MASON Stands for Multi-Agent Simulator Of Neighborhoods... or Networks... or something...
Homepage: http://cs.gmu.edu/~eclab/projects/mason.