1. A single computer system, comprising: one or more processors; and a memory coupled to the one or more processors, wherein the memory is configured to store program instructions executable by the one or more processors to implement a simulator, wherein the simulator comprises: a simulation engine configured to generate concurrent user activity for a plurality of different simulated users; and a first browser configured to receive the user activity from the simulation engine and send Internet requests to one or more destinations according to the user activity so that the Internet requests from the browser appear to be from a plurality of different concurrent users.