google_shortcuts.user.js

// ==UserScript==
// @name Google shortcuts
// @include http://www.google.*/search*
// @include https://encrypted.google.com/search*
// @description Adds useful shortcuts to Google search results page
// ==/UserScript==

(function(){
    var results = document.querySelectorAll("#ires>ol>li.g.w0");
    var activated = false;
    var indexStyles = "background: #1111cc;" +
                      "font: 10px verdana, arial, sans;" +
                      "width: 12px; height: 12px;" +
                      "display: block; text-align: center;" +
                      "color: white; float: left;" +
                      "margin-right: 5px;" +
                      "-webkit-border-radius: 6px;"; 

    activateShortcuts = function() {
        if (!activated) {
            activated = true;
            for (var i = 0; i < results.length && i < 9; i++) {
                var index = document.createElement("span");
                index.setAttribute("style", indexStyles);
                index.setAttribute("class", "index");
                index.innerHTML = i + 1;
                var header = results[i].firstChild;
                header.insertBefore(index, header.firstChild);
            }
        }
    };

    disableShortcuts = function() {
        if (activated) {
            activated = false;
            var elems = document.querySelectorAll("span.index");
            for (var i = 0; i < elems.length; i++) {
                elems[i].parentNode.removeChild(elems[i]);
            }
        }
    };

    openLink = function(index) {
        window.location = results[index].querySelector("h3.r>a.l").href;    
    };

    document.addEventListener("keydown", function(event) {
        if (document.activeElement.value) return;
        if (event.keyCode == 79) { // o
            if (!activated) {
                activateShortcuts();
            } else {
                disableShortcuts();
            }
        } else if (event.keyCode >= 49 && event.keyCode <= 57 &&
            (event.keyCode - 49) < results.length) { // 1-9
            openLink(event.keyCode - 49);
        } else {
            disableShortcuts();
        }
    });
 })()