This paper describes JSIM: the simulation module of the Java Modelling Tools (JMT), an open-source fully-portable Java suite for capacity planning studies. The simulator has been purposely developed to help both unexperienced and advanced users. Most of the difficult decisions that are needed in order to run simulations properly, such as the detection of the transient part of samples to be discarded, have been automated. The tool also provides guidance over the graphical design of the network and over the analysis and the plot of the results. What-if parametric analyses for parametric evaluation of complex systems are supported. Several features that increase the generality of the applica- tions to capacity planning studies are provided, among them fork-join service centers, regions with finite capacity, state- dependent routing algorithms, priority classes and import of real workload distributions from log files.