We have recently constructed a model, and carried out an analysis, of a concurrent extension to an object-oriented language at a level of abstraction above threads. The model was constructed in CSP. We subsequently found that existing CSP tools were unsuitable for reasoning about and analysing this model, so it became necessary to create a new tool to handle CSP models: CSPsim. We describe this tool, its capabilities and algorithms, and compare it with the related tools, FDR2 and ProBE. We illustrate CSPsim's usage with examples from the model. The tool's on-the-fly construction of successor states is important for exhaustive and non-exhaustive state exploration. Thus we found CSPsim to be particularly useful for parallel compositions of components with infinite states that reduce to finite-state systems.