Con select.

codice HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<script type="text/javascript">
function pippo() {
        var c2 = document.getElementById("riga");
        if (document.getElementById("campo1").value == '--') {
            document.getElementById("campo2").value ="";
            c2.style.visibility = 'hidden';
        } else {
            if(c2.style.visibility == 'hidden') {
                  c2.style.visibility = 'visible';
            }
        }
}
</script>
<title></title>
</head>
<body>
<table>
    <tr>
       <td>Campo 1</td>
      <td><select id="campo1" onchange="pippo()">
           <option value="--">-- Seleziona --</option>
           <option value="1">Uno</option>
         <option value="2">Due</option>
         <option value="3">Tre</option>
        </select>
      </td>
    </tr>
    <tr id='riga' style="visibility:hidden">
        <td>Campo 2</td>
      <td><input type="text" id="campo2"></td>
       </tr>
</table>
</body>
</html>