podria ser algo asi:
<script language=javascript>
<!--
function mostrar(d) { d.style.position=''; d.style.visibility='visible' }
function ocultar(d) { d.style.position='absolute'; d.style.visibility='hidden' }
//-->
</script>
<b onmouseover="mostrar(document.getElementById('mica pa'))" onmouseout="ocultar(document.getElementById('micap a'))">posa el mouse encima!</b>
<div id="micapa" style="position:absolute;visibility:hidden">
aqui el contenido
</div>