Moduleco is a "modular "multi-agent" platform, designed for to simulate markets and organizations, social phenomenons and population dynamics."
In addition to being a resource for agent-based modeling information, SwarmWiki is the home of Swarm, one of the original agent-based simulation programs. Swarm is a "multi-agent software platform for the simulation of complex adaptive systems. In the Swarm system the basic unit of simulation is the swarm, a collection of agents executing a schedule of actions. Swarm supports hierarchical modeling approaches whereby agents can be composed of swarms of other agents in nested structures. Swarm provides object oriented libraries of reusable components for building models and analyzing, displaying, and controlling experiments on those models." An in-depth description of Swarm can be found here