Agent Based Simulation Engines

COS Sim - Traffic flow simulation
http://code.google.com/p/cos-sim/

SimZombie - This looks like fun
http://code.google.com/p/simzombie/


XMLisp - Seems like a simulation toolkit or a simple game engine. 
http://code.google.com/p/xmlisp/


MASON - This one looks quite well developed.
http://cs.gmu.edu/~eclab/projects/mason/

RePast - Looks well developed and flexible. 
http://repast.sourceforge.net/

AScape
http://ascape.sourceforge.net/
This is yet another Java based engine for doing 2d simple agents in a simple world.  This is even more rigid as it uses a latice world.

Swarm
http://www.swarm.org/index.php/Main_Page
http://www.swarm.org/index.php/Swarm_main_page
http://www.swarm.org/images/b/bb/MinarEtAl96.pdf
Objective-C.... mmmk.  Reading the ideas page gives you a bit of an idea of the amount of baggage that this system is dragging around with it.  It looks like a very nice package but its got bloat and is trying to be all sorts of things to too many people.  I think once you spilt your efforts across one or more languages... its all over... the efforts of the community get sucked up by having to duplicate everything and trying to support new work with a foot on both sides of the langauge barrier.  It just buries the pace of evolution. I think rob the system for all the good ideas and start again clean.  .NET across Mono is probably the easy solution with some front end work done in HTML5 for simple cross platform support.

Breve
http://www.spiderland.org/
This looks like a nice little package. Fast and easy to set up with a good graphics engine already in place. Has a good set of docs and examples.  Spot the teaching tool. I could do some things with this.  There is some framework already in place but some you would need to implement.  Have not looked at the source yet.


The Mega List of Agent Modelling Software
http://en.wikipedia.org/wiki/Comparison_of_agent-based_modeling_software