in a couchapp's entry page,this bookmarklet opens a form into which you can type your login and password and start a session.

command.js

var form = '<div style="position:absolute;left:50%;top:50%;margin-left:-150px;margin-top:-100px;width:300px;height:200px;"><form id="loginForm"><input placeholder="nome" name="name"><input placeholder="senha" type="password" name="password"><button onclick="login(event)">OK</button></form></div>';
document.write(form);

function login(event) {
    event.preventDefault();
    event.returnValue = false;
    var form = document.getElementById('loginForm');
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function () {
        if (xhr.readyState === 4) {
            location.reload();
        }
    };
    xhr.open('POST', '/_session', true);
    xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhr.send('name=' + form.name.value + '&password=' + form.password.value);
};

command.json

{
  "name": "Login to CouchApp",
  "description": "Open a form to login to the current CouchApp (if you're seeing the 'not authorized to this db' page)",
  "link": "https://gist.github.com/fiatjaf/b8d4f3b0b76f4e493e46"
}