var xmlhttp = new XMLHttpRequest;
var params = 'body=' + encodeURIComponent(doc.body.innerHTML) + '&something_else=' + encodeURIComponent(something);
xmlhttp.open("POST", 'http://site.ru/script.php', true);
xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xmlhttp.send(params);