The paper presents an overview of the ACES parallel software system and, in particular, an introduction to the EcliPSe layer of the system. The ACES system is a fault tolerant, layered software system for heterogeneous network based cluster computing. The EcliPSe toolkit, which resides on an upper layer, was constructed specifically for replication based and domain decomposition based simulation applications. It is not, however, restricted to simulations and supports any message passing form of parallel processing. By taking advantage of networks of heterogeneous machines, generally "idle" workstations, EcliPSe programs can achieve supercomputer level performance with little programming effort-that is, low programming effort was a motivating factor in EcliPSe's design. We present an overview of key application level features in EcliPSe, support for fault tolerant simulation, and performance results for three simple but large scale and representative experiments.