<script language="javascript">
function f1(event){
    popup.style.left=event.clientX+ 5 ;
    popup.style.top=event.clientY+ 15 ;
    popup.style.display='';
}
function f2(){
    popup.style.display='none';
}
</script>

..............

<div style="border: solid 1px gray;" onmousemove="f1(event)" onmouseout="f2()">
    Mouse here
</div>
<div id="popup" style="position: absolute; display: none; width: 100px; height: 40px; background: white; border: solid 1px silver;">
    Popup
</div>