In this paper we present the design of a simulator platform called FUSE (Fast Universal Simulator Engine). The term Universal means that the Engine can be adapted easily to different domains and be used for varying simulation needs, although our main target is simulation of distributed algorithms in distributed computing environments. The Engine is Fast in the sense that the simulation overhead is minimal and very large systems can be simulated. We discuss the architecture and the design decisions that form the basis of these features. We also describe the functionality that is provided to its users (e.g., monitoring, statistics, etc.).