This paper documents the features and the design of XMTSim, the cycle-accurate simulator of the Explicit Multi-Threading (XMT) computer architecture. The Explicit Multi-Threading (XMT) is a general-purpose many-core computing platform, with the vision of a 1000-core chip that is easy to program but does not compromise on performance. XMTSim is a primary component in its publicly available toolchain along with an optimizing compiler. Research and experimentation enabled by the toolchain played a central role in supporting the ease-of-programming and performance aspects of the XMT architecture. The compiler and the simulator are also important milestones for an efficient programmer's workflow from PRAM algorithms to programs that run on the shared memory XMT hardware. This workflow is a key component in accomplishing the goal of ease-of-programming and performance. The applicability of the XMT simulator extends beyond specific XMT choices. It can be used to explore the much greater design space of shared memory many-cores by system researchers or by programmers. As the toolchain can practically run on any computer, it provides a supportive environment for teaching parallel algorithmic thinking with a programming component.