This paper describes a novel software application that assists in understanding the process of estimating the state of a linear dynamic system based on noisy output signal measurements. An interactive Java tool, based on the Kalman Filter, is described. This consists of two main parts, a simple one-dimensional filter and a multi-dimensional filter tool. The user can set all input parameters through a single interface or by following a series of guided steps. Raw measurement data can be generated automatically or inserted manually by the user and the results of estimating the true system state are then presented as a series of graphs in real-time. The application is described through the use of two simple examples. Such an application could be used to teach signal and systems engineering.