This paper describes MTSim, an extensible, customizable simulation platform for the Modechart toolset (MT). MTSim provides support for "plugging in" user-defined viewers useful in simulating system behavior in different ways, including application-specific ways. MTSim also supports full user participation in the generation of simulations by allowing users to inject events into the execution trace. Moreover, MTSim provides monitoring and assertion checking of execution traces and the invocation of user-specified handlers upon assertion violation. This paper also introduces an MTSim component called WebSim, a suite of simulation tools for MT, and an application-specific component of MTSim which displays the cockpit of an F-18 aircraft and which responds to user inputs to model a bomb release function.