Connexion

<a  id="login">Login <span><input name="login"/><br/><input type="submit" name="connexion" value="Se connecter" /></span></a>

a#login span{display:none;} 
a#login:hover span{display:block;position:absolute;margin-top:20px;}

  function montre(elem){
  document.getElementById(elem).style.display="block";
  }
  function cache(elem){
  document.getElementById(elem).style.display="none";
  }

<ul>
<li onmouseover="montre('bloc1');" onmouseout="cache('bloc1');">Connexion
<div id="bloc1" style="display:none; width:200px;height:200px;border:1px solid #4A4B4B;position:absolute;margin-top:-5px;padding:5px;>
<form method="post" action="#" id="recherche">
<label>Identifiant</label>
<input type="text" value=""><br/>
<label>Mot de passe ( oublié ? ) </label>
<input type="text" value="">
<input type="submit" value="go">
</form>
</div>
</li>
</ul>