[Rise]

danik.js, так вас устроит

document.getElementById('but').onclick = function() {
	var ajax = new XMLHttpRequest();
	var params = 'check=' + +document.getElementById('checkbox_jax').checked;
	ajax.open('POST', '/auth.php', true);
	ajax.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
	ajax.onreadystatechange = function() {
		if (this.readyState == 4 && this.status == 200) {
			document.getElementById('res_log').innerHTML = this.responseText;
		}
	}
	ajax.send(params);
	return false;
}