Codice:
<a href="#" onmouseover="javascript:mostra();" onmouseout="javascript:nascondi();">Giochi</a> <span id="testo" style="display: none;">su Nipogames.com</span>
mancavano 2 lettere 
EDIT: comunque io farei così:
Codice HTML:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento senza titolo</title>
<script language="JavaScript">
<!--
function mostra(obj){
document.getElementById('testo_'+obj).style.display='';
}
function nascondi(obj){
document.getElementById('testo_'+obj).style.display='none';
}
//-->
</script>
</head>
<body>
<a href="#" onmouseover="mostra(1);" onmouseout="nascondi(1);">Giochi</a> <span id="testo_1" style="display: none;">su Nipogames.com</span><br/>
<a href="#" onmouseover="mostra(2);" onmouseout="nascondi(2);">blabla</a> <span id="testo_2" style="display: none;">su blablabla</span>
</body>
</html>