trello-id.user.js

// ==UserScript==
// @name           Trello ID
// @author         Jakub Kuźma
// @version        0.1.1
// @description    Displays IDs on the cards
// @include        https://trello.com/*
// ==/UserScript==

var main = function () {
    setInterval(function () {
        $(".list-card:not(:has(.list-card-id))").each(function (i, listCard) {
            var href = $(listCard).find("a.list-card-title").attr("href"),
                id = href.match(/\d+$/)[0];

            $(listCard).append('<div class="list-card-id">#' + id + '</div>');
        });
    }, 1000);
};

var script = document.createElement("script");
script.type = "text/javascript";
script.textContent = "(" + main.toString() + ")();";
document.body.appendChild(script);