The Simultaneity Spacetime Diagram model uses light-trajectories to show the effect of relative motion when observing (recording) events in special relativity.   In the default scenario, an explosion (an event) at the center of a right-moving stick occurs at t=0 and the arrival of the explosion light signal at each end is recorded.  The arrival event at the left end occurs before the arrival event at the right end because the stick is moving.  How do the location and time of these events change if they are observed in a reference frame (the Other Frame) in which the stick is stationary?  
 
The Simultaneity Spacetime Diagram model was written for the study of special relativity the Lorentz transformation using spacetime diagrams and is a supplemental simulation for an article by Sebastien Cromier and Richard Steinberg "The Twin Twin Paradox: Exploring Student Approaches to Understanding Relativistic Concepts" in the The Physics Teacher 48 (9), 598-601 (2010).  Initial conditions, such as the locations of the explosion and the detectors, can be adjusted by dragging before the simulation is run. The slider can be used to change the speed of the stick.  A third view shows the stick and the wavefront in space.  More importantly, a checkbox allows users to compare the predictions of Galilean and special relativity in order to observe how the assumption of a constant speed of light leads to the relativity of simultaneity.
 
The EJS JavaScript version of the Simultaneity Spacetime Diagram simulation was adapted from the original Java version of that program.