Tooltip.js

jQuery:
tip.init(object) #tooltip will be taken from object "helper" attribute
img src="..." helper="..."
tip.init('img[helper]')

var tip = {
init : function(e){
   $(e).bind("mouseenter", this.createTip);
   $(e).mouseleave(function(){$("#tip").remove()});
   $(e).click(function(){$("#tip").remove()});
   $(e).mousemove(function(e){$("#tip").css({left:e.pageX+30,
                                             top:e.pageY-16})});
},
createTip : function(e) {
    var obj=$(e.currentTarget),
        title=$.trim(obj.attr("helper"));
    if(title.length>0){        
       return $("#tip").length === 0 ?
          $("<div>").html("<span>"+$.trim(obj.attr("helper"))+"</span>").
                     attr("id","tip").
                     css({left:e.pageX+30,
                          top:e.pageY-16,
                          position:'absolute',
                          border:'1px solid #FFE222',
                          background:'#FFFBC2',
                          color:'#514721',
                          padding:'5px 10px',
                          textTransform:'lowercase',
                          fontVariant:'small-caps',
                          zIndex:9000}).
                     appendTo("body") : null;
        } else {return false}
    }
};

Prototype:
tip.createTip(container,object) #tooltip will be taken from object "title" attribute
<div id="container">
<img src="..." title="..." />
</div>
tip.createTip($('container'),'img[title]')

var tip = {
 title  : '',
 opacity: .8,
 marginX: 30,
 marginY: -46,
 position : function(i,e){
     return i.setStyle({left:e.clientX+this.marginX+'px',
                        top:e.clientY+this.marginY+'px'})
 },
 enter : function(i){
     var tip=this;
     $(i).observe(antHill.events.menter,function(e){
         var div,obj=$(e.currentTarget);
         tip.title=obj.readAttribute("title");
         obj.writeAttribute("title",'');
         if($("tip")===null){
             div=new Element('div',{id:'tip'});
             div.addClassName('tooltip').
                 setOpacity(tip.opacity).
                 innerHTML="<span>"+tip.title+"</span>";
             tip.position(div,e);
             $($$('body')[0]).insert({bottom:div});
         }
         Event.stop(e)}.bindAsEventListener($(i)));
 },
 leave : function(i){
     $(i).observe(antHill.events.mleave,function(e){
         $("tip").remove();
         $(i).writeAttribute("title",tip.title);
         Event.stop(e)}.bindAsEventListener($(i)));
 },
 move : function(i){
     $(i).observe(antHill.events.mmove,function(e){
         antHill.tip.position($("tip"),e);
         Event.stop(e)}.bindAsEventListener($(i)));
 },
 init : function(i){
     this.enter(i);
     this.leave(i);
     this.move(i);
 },
 createTip : function(c,i){c.select(i).
                             each(function(i){tip.init(i)})}
};