Stackless Python with the uthread module is amazingly powerful. It took only a few hours of development to create a nice and rather complex population simulation, and it was rather fun to do so. My underpowered little Gateway laptop (with 196MB of RAM) scales easily to tens of thousands of agents, running in as many threads. I'll be maintaining this simulation as a kind of hobby project. The full source is available from www.valdyas.org/python/worldsim.html.