The Recursive Porous Agent Simulation Toolkit (Repast) is a widely used free and open-source, cross-platform, agent-based modeling and simulation toolkit. Repast has multiple implementations in several languages (North, Collier & Vos 2006) and built-in adaptive features, such as genetic algorithms and regression.
Repast was originally developed by David Sallach, Nick Collier, Tom Howe, Michael North and others at the University of Chicago.
Features
variety of agents and examples
fully object oriented
fully concurrent discrete event scheduler
built-in simulation results logging and graphing tools (North et al. 2007)
allows users to dynamically access and modify agents and model at run time
libraries for genetic algorithms, neural networks, etc.
built-in systems dynamics modeling
social network modeling tools
integrated geographical information systems (GIS) support
implemented in Java, C#, etc.
supports Java, C#, Managed C++, Visual Basic.Net, Managed Lisp, Managed Prolog, and Python scripting, etc.
is available on virtually all modern computing platforms