Here is a code sample that converts text in each paragraph to upper case when you place a mouse over it and returns to the original value on mouseout. This may be stupid but it does illustrate a few DOM techniques including how to get a reference to all page elements, and how to create additional properties on an object.  Works only in Firefox at this time.

  function ucListener(){

         //save the existing text in a custom property
        this.firstChild.oldValue = this.firstChild.nodeValue;
        this.firstChild.nodeValue = this.firstChild.nodeValue.toUpperCase();
    }

    function ucListenerOut(){
            this.firstChild.nodeValue = this.firstChild.oldValue;
    }

    function installAllListeners() {
        var elements = document.getElementsByTagName(’*');
        var i = elements.length;
        while(i–>0){
            elements[i].addEventListener(’mouseover’, ucListener, false);
            elements[i].addEventListener(’mouseout’, ucListenerOut, false);
        }
    }