Trello_Tweaks.user.js

// ==UserScript==
// @name        Trello Tweaks
// @namespace   http://timmorgan.org
// @include     https://trello.com/*
// @version     1
// @grant       none
// ==/UserScript==

// press Q to show cards not assigned to anyone

var showingUnassignedCards = false;

function toggleUnassignedCards() {
  if (showingUnassignedCards)
    $('.list-card').removeClass('hide');
  else
    $('.list-card').filter(function(i, c) { return $(c).find('.list-card-members *').length > 0 }).addClass('hide');
  showingUnassignedCards ^= true;
}

function keyup(e) {
  if (e.key == 'Q')
    toggleUnassignedCards();
}

document.addEventListener('keyup', keyup, false);