While I have blogged about creating geographically explicit agent-based models with Repast, MASON, NetLogo and several other toolkits before, I recently came across an open source toolkit called Gis & Agent-based Modelling Architecture or GAMA for short. GAMA is developed by UMMISCO and to quote from the site:
"GAMA is a simulation platform, which aims at providing field experts, modelers, and computer scientists with a complete modeling and simulation development environment for building spatially explicit agent-based simulations."
The site offers a series of tutorials, supporting documents and publications which show the potential of GAMA. The movie below gives an excellent overview of what GAMA can do.