Edit in JSFiddle

/**
*	Exemplo de script para preenchimento automatico
*	de formulários.
*/
document.getElementById('preencher').addEventListener("click", 
   function () {
      wForms = document.forms.length;
      for(x=0; x<=wForms-1; x++) {
         wElems = document.forms[x].elements.length;
         for(i=0; i<=wElems-1; i++) {
            wElement = document.forms[x].elements[i];
            wType = wElement.getAttribute("type");
            wName = wElement.getAttribute("name");
            if(wType=="text" || wType=="password") {
               wElement.value = wName;
            }
            if(wType=="email") {
               wElement.value = "[email protected]";
            }
            if(wType=="radio" || wType=="checkbox") {
               wElement.click();
            }
         }
      }
    }
 );

<p>
Exemplo de script para preenchimento automatico
de formulários.
</p>
<form id="teste" onsubmit="" method="post">
   <p>Nome: <input name="input_nome" type=text></p>
   <p>E-mail: <input name="input_email" type=email></p>
   <p>Telefone: <input name="input_telefone" type=text></p>
   <p>Senha: <input name="input_senha" type=password></p>
   <p>
      <input name="radios[]" type="radio" value="1"> opção 1<br>
      <input name="radios[]" type="radio" value="2"> opção 2<br>
      <input name="radios[]" type="radio" value="3"> opção 3<br>
      <input name="radios[]" type="radio" value="4"> opção 4<br>
   </p>
   <p>
      <input name="checkboxes" type="checkbox" value="1"> checkbox 1<br>
      <input name="checkboxes" type="checkbox" value="2"> checkbox 2<br>
      <input name="checkboxes" type="checkbox" value="3"> checkbox 3<br>
   </p>
   <input id="preencher" type="button" value="preencher">
</form>

body * {
  font-family: Tahoma,Verdana,Arial,Sans;
  font-size: 0.95em;
}
input {
  padding: 2px 5px;
  width: 300px;
}
input[type=checkbox],
input[type=radio] {
  width: 18px;
  height: 18px;
}
input[type=button] {
  width: auto;
  padding: 10px 15px;
}