If you can forgive me for departing from the tradition established by 11 comments all talking about the card suit in the title, Agents.jl is a powerful package. I just used it to research a problem in simulated biological evolution, and it did the job wonderfully. You can define the behavior of your agents and their interaction with the environment at a high level, and Agents.jl will take care of the details, such as enforcing boundary conditions, creating ensembles, etc. And it’s fast, and visualization is easy. (I haven’t actually read the article yet.)