Used this to track down a crazy JS bug one time. Still makes me smile.

event_inspector.js

var eventColors = {
  mouseover: 'red',
  mouseout: 'blue',
  click: 'yellow'
}

var highlight = function(event) {
  Event.stop(event);
  element = Event.element(event);
  Element.setStyle(element, {backgroundColor: eventColors[event.type]});
  setTimeout(Element.setStyle.bind(null, element, {backgroundColor: ''}), 200);
}

Event.observe(window, 'load', function() {
  Event.observe(document.body, 'mouseover', highlight.bindAsEventListener());
  Event.observe(document.body, 'mouseout', highlight.bindAsEventListener());
  Event.observe(document.body, 'click', highlight.bindAsEventListener());
});