NetLogo is a free agent-based modelling software developed at the Centre for Connected Learning and Computer-based Modelling at NorthWestern University, Illinois, US. It offers a multi-agent programmable modelling environment.
A beta release of Vensim 5.9e is now available in Mac format for OSX.
Insight Maker is a free online systems modelling software. The online software provides a variety of systems modelling tools and is particularly strong in Systems Dynamics and Agent-based modelling. The website also contains many useful sample models and simulations. The cost is your model/simulation is available to Insight Maker.