Research in self-adaptive systems often uses web applications as target systems, running the actual software on real web servers. This approach has three drawbacks. First, these systems are not easy and/or cheap to deploy. Second, run-time conditions cannot be replicated exactly to compare different adaptation approaches due to uncontrolled factors. Third, running experiments is time-consuming. To address these issues, we present SWIM, an exemplar that simulates a web application. SWIM can be used as a target system with an external adaptation manager interacting with it through its TCP-based interface. Since the servers are simulated, this use case addresses the first two problems. The full benefit of SWIM is attained when the adaptation manager is built as a simulation module. An experiment using a simulated 60-server cluster, processing 18 hours of traffic with 29 million requests takes only 5 minutes to run on a laptop computer. SWIM has been used for evaluating self-adaptation approaches, and for a comparative study of model-based predictive approaches to self-adaptation.