If you don't mind limiting your scripts to IE5.5 or greater, then...
<script language="javascript">
<!--
var xmlHTTP = new ActiveXObject("Microsoft.XMLHTTP");
var bIgnoreWaitforserverresponse= false;
var strUrltotest = "http://localhost/foo_bar.html";
function sendtoserver(strUrl, strPostvars) {
if ( typeof xmlHTTP == "object" ) {
xmlHTTP.open("POST", strUrltotest , bgnoreWaitforserverresponse);
xmlHTTP.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
var strCmd = "x=y&b=z"; // POST vars go here
xmlHTTP.send(strCmd);
}
}
// -->
</script>
or.. for a more generic approach try:
http://dynarch.com/mishoo/rpc.epl