This depends what you mean by economic simulations -- if you are looking for general equilibrium models, Dynare is pretty popular, although many people would just code from scratch, and there are various codes here and there.<p>Someone else mentioned agent-based models; these are interesting, but there is a reason why there has been some (now decreasing, as models develop) skepticism towards them -- for example, quite often we see rather dumb simulated agents, who do not react to environment and do not learn from their repeated mistakes (for example, by not adjusting their behavior to tax rates...)