Visual Event is a Javascript bookmarklet which provides debugging information about events that have been attached to DOM elements. Visual Event shows:

Which elements have events attached to them
The type of events attached to an element
The code that will be run with the event is triggered
The source file and line number for where the attached function was defined (Webkit browsers and Opera only)

In addition to being useful for debugging your own code, Visual Event can be used as an educational tool, showing how many web-sites have been authored.