Tampermonkey script that allows you to hover over a user's name to view his karma while viewing comments on hackernews.

hoverAndGrabKarma.js

// ==UserScript==
// @name       Hover Over And Grab Karma
// @namespace  http://use.i.E.your.homepage/
// @version    0.1
// @description  While viewing comments, allows you to hover over a username to view his karma
// @match      https://news.ycombinator.com/item?*
// @copyright  2012+, You
// ==/UserScript==

var allLinks = document.querySelectorAll("a");
allLinks = Array.prototype.slice.call(allLinks); // convert nodeList to array
var userLinks = allLinks.filter(function(link) {
    return link.href.indexOf("user?id") !== -1;
});
userLinks.forEach(function(link) {
    link.onmouseover = function() { addKarmaInfo(link) };
});

function addKarmaInfo(link) {
    var onSuccess = function(response) {
        var karma = response.match(/karma:.{9}(\d+)/)[1];
        addUserInfo(link, " (karma: " + karma + ") ");
    };
    grabKarma(link.href, onSuccess);
}

function grabKarma(url, onSuccess) {
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function() {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        onSuccess(xmlhttp.response);
    }
    xmlhttp.open("GET",url,true);
    xmlhttp.send();
}

// Adds or updates text node after the username link
function addUserInfo(afterLink, textToAdd) {
    
    var nextSibling = afterLink.nextSibling;
    
    // span doesn't exist yet, create it
    if (!nextSibling || nextSibling.nodeName !== 'SPAN') {
    	repSpan = document.createElement('span');
    	repSpan.setAttribute("class", "reputation");
        afterLink.parentNode.insertBefore(repSpan, nextSibling);
    }
    // now update it
    repSpan = afterLink.nextSibling;
    repSpan.innerHTML = textToAdd;     
    console.log(afterLink.nextSibling);
}