Userscripts.org

Works with new grooveshark:

// ==UserScript==
// @name Grooveshark Ad Remover
// @namespace http://itsjareds.leadhoster.com/
// @description Removes the side banner advertisements on Grooveshark.
// @author KnifeySpooney (http://spogg.com/KnifeySpooney, http://wurbo.com) -- updated by syn
// @version 1.1
// @include http://listen.grooveshark.com/*
// ==/UserScript==

var sidebar = document.getElementById("sidebar");
var wrap = document.getElementById("mainContentWrapper");

var removeAd = function() {
if (wrap && sidebar) {
wrap.style.marginRight = "0px";
sidebar.style.display = "none";
}
}

wrap.addEventListener("DOMSubtreeModified", removeAd, true);
removeAd();