Repast

Repast (Recursive Porous Agent Simulation Toolkit) is an free open source agent-based modelling toolkit. The Java-implementation of its version 3 (Repast J) is used as part of the SE (Simulation Environment ) subsystem of the OCOPOMO toolbox, where is drives the DRAMS-based simulation models by providing scheduling, mathematics, graph plotting and other functionality. Repast 3 implementations are additionally available for Microsoft.Net (Repast.Net) and Python (Repast Py).
Repast 3 has recently been superseded by Repast Simphony. This version is based on Java technologies and brings a lot of new features which support the interactive development of flexible simulation models. Supported modelling languages include Java, Groovy, a Logo dialect as well as flow charts.

See http://repast.sourceforge.net/