A couple functions for creating AJAX requests.

ajax-util.js

/**
* The postAjaxRequest and getAjaxRequst take arguments for a callback function, url, and param string. 
* Both requests return the response text using "this" inside of the callback function.
*/

// POST request
function postAjaxRequest(callback, url, args) {
  var contentType = 'application/x-www-form-urlencoded';
  var ajax = new createAjaxObject(callback);
  if (!ajax) {
    return false;
  }
		
  ajax.open('POST', url, true);
  ajax.setRequestHeader('Content-type', contentType);
  ajax.setRequestHeader('Content-length', args.length);
  ajax.setRequestHeader('Connection', 'close');
  ajax.send(args);
  return true;
}
	
// GET request
function getAjaxRequest(callback, url, args) {
  // Ensure cached call is not used
  var nocache = '&nocache=' + Math.random() * 1000000;

  var ajax = new createAjaxObject(callback);
  if (!ajax) {
    return false;
  }

  ajax.open('GET', url + '?' + args + nocache, true);
  ajax.send();
  return true;
}
	
// Create the AJAX object. Nested Try/Catch blocks to accomodate different browsers
function createAjaxObject(callback) {
  var ajax;
  try {
    ajax = new XMLHttpRequest();
  }
  catch (ex1) {
    try {
      ajax = new ActiveXObject("Msxm12.XMLHTTP");
    }
    catch (ex2) {
      try {
        ajax = new ActiveXObject("Microsoft.XMLHTTP");
      }
      catch (ex3) {
	ajax = false;
      }
    }
  }
		
  if (ajax) {
    ajax.onreadystatechange = function() {
      if (this.readyState == 4 && this.status == 200 && this.responseText != null) {
        callback.call(this.responseText);
      }
    };
   }
  return ajax;
}