Water resources management is one of the most challenging problems of the water sector. The complexity of this problem is mostly attributed to the dynamic nature of the natural hydrosystems, of which the inputs are characterized by stochasticity. The assessment of the impact of alternative decisions on such a system requires a probabilistic approach. It is evident that this is a very demanding task even for a single-reservoir system, not to mention multi-reservoir systems. For this reason, computer codes have been developed since the early 1970s to help with this procedure. Nowadays, these codes have evolved into decision support platforms that offer a user interface to assist the pre-processing and the post-processing. However, the majority of the available tools are commercial and require a considerable amount of money for a licence. Thankfully, a few freeware tools that are based on the simulation–optimization method and with a modern user interface exist. The objective of this study is to assess the performance of these freeware tools. The assessment is based both on an analysis of the theoretical basis and on an application of the models on a simplified representation of the Athens water supply system.