Fijate si te sirve algo así:
Código PHP:
<!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 sin título</title>
<style>
*{margin:0; padding:0}
#lista, #lista input{width:200px}
#lista dd{position:absolute; visibility:hidden;width:200px; background-color: #DFDFDF}
#lista ul{ margin-top:2px; border:1px dashed #000; border-bottom:none}
#lista li{ list-style: none; text-align:center; border-bottom:1px dashed #000; cursor: pointer}
</style>
</head>
<body>
<form>
<dl id="lista">
<dt><input id="algo" type="text" onfocus="document.getElementById('lista').getElementsByTagName('dd')[0].style.visibility='visible'" onkeypress="document.getElementById('lista').getElementsByTagName('dd')[0].style.visibility='hidden'" /></dt>
<dd><ul><li onclick="document.getElementById('algo').value=this.innerHTML;document.getElementById('lista').getElementsByTagName('dd')[0].style.visibility='hidden'">MTS</li><li onclick="document.getElementById('algo').value=this.innerHTML;document.getElementById('lista').getElementsByTagName('dd')[0].style.visibility='hidden'">C/U</li><li onclick="document.getElementById('algo').value=this.innerHTML;document.getElementById('lista').getElementsByTagName('dd')[0].style.visibility='hidden'">X1000</li></ul></dd>
</dl>
más cosas
</form>
</body>
</html>