EKB just spotted this browsing SourceForge:

 Description: "EcoLab is both the name of a software package and a
               research project that is looking at the dynamics of
               evolution. EcoLab the software package is now a fairly
               complete agent-based simulation system." (From web site)
 Updated: Dec 2006
 Contact: See web site

From the description:

  • The model is implemented as a C++ object. Support for more advanced data structures and algorithms are available through C++'s standard library.
  • A scripting language TCL can access the model's methods and instance variables, allowing experiments to be set up dynamically at runtime
  • The model can be run in GUI mode through the use of Tk and BLT widgets. Various graphs and histogramming tools are available for use.