alert(document.domain);

location.host

function getHost(url)
{
    var a = document.createElement('a');
    a.href = url;
    return a.hostname;
}

location = {
  host: "stackoverflow.com",
  hostname: "stackoverflow.com",
  href: "http://stackoverflow.com/questions/2300771/jquery-domain-get-url",
  pathname: "/questions/2300771/jquery-domain-get-url",
  port: "",
  protocol: "http:"
}

location.host

location.protocol + "//" + location.host

document.domain

//If url is something.domain.com this returns -> domain.com
function getDomain() {
  return window.location.hostname.replace(/([a-zA-Z0-9]+.)/,"");
}