Simple A-Life simulation ala Cellular Automaton with some mixture of artifical genetics.
The world is again 2-D grid with "creatures" represented as boxes on the grid. Creatures have some genetic code, which tranferes itsef into set of simple commands, which guide their life on the grid. And of course creatures envolves through random changes of their genome. Old trick, but still quite interesting. You can see what randomly (though initial seeding and evolution) generated code is most successfull.
Source code is available via CVS from this SF project.