Poor mans Javascript console

Quick and dirty way of testing Javascript live on a website in browsers without Firebug:

<form onsubmit="try {
    $('output').innerHTML = $F('input') + ' => ' + eval($F('input'));
  } catch (e) {
  return false">
  <div id="output"></div>
  <input id="input" name="input" value="" on="" />

Requires Prototype, but could probably be rewritten as pure Javascript pretty easily. Could also be turned into a bookmarklet, I guess.