I also recommend NetLogo. It is an IDE+environment+programming language based on logo (which was based on Lisp) which lets you build multi-agent models extremely fast. I have found that I can reproduce (simulate) algorithms from research articles in a couple of hours, algorithms that would have taken weeks to implement with other libraries.
PHP and Java developers should take a look at KATO.
If you are an evolutionary economist you can also check Laboratory for Simulation Development (LSD).
On the modelling side, have a look at FAML, an agent-oriented modelling language. This is a pretty academic paper, but it may help depending on your interests: http://ieeexplore.ieee.org/xpl/freepre_abs_all.jsp?isnumber=4359463&arnumber=4967615
I also recommend NetLogo. It is an IDE+environment+programming language based on logo (which was based on Lisp) which lets you build multi-agent models extremely fast. I have found that I can reproduce (simulate) algorithms from research articles in a couple of hours, algorithms that would have taken weeks to implement with other libraries.
You can check some of my models at this page.