O'Reilly's 'Code Zoo' project has a page of open-source software for simulation. Code Zoo is a directory of re-usable open source components for programmers.
At the moment most of the code on simulation seems to be in Java. One example is The Aglets Software Development Kit (ASDK), "a framework and environment for researching and developing mobile agents."
Another is RePast, "a framework for creating agent based simulations". The RePast website says:
"Our goal with Repast is to move beyond the representation of agents
as discrete, self-contained entities in favor of a view of social
actors as permeable, interleaved, and mutually defining; with
cascading and recombinant motives. We intend to support the
modeling of belief systems, agents, organizations, and institutions
as recursive social constructions."
As someone who still thinks Java is an island, I have no way of evaluating it; but the Code Zoo is worth watching. It will concentrate on code in Java, Python and Ruby.