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.
Vensim is simulation software for modelling the real world and improving the performance of real systems. It offers classic system dynamics modelling along with flight simulators in a range of products from the free academic and personal use versions to professional products capable of modelling the most complex situations.
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.
A beta release of Vensim 5.9e is now available in Mac format for OSX.