基本用法：
代码如下：

var ajax = new AjaxObj(url);
ajax.addListener(200, function(r){
alert(r);
});
ajax.send();

也可以连续调用：
代码如下：

var ajax = new AjaxObj(url).addListener(200, function(r){
alert(r);
}).send();

另外还支持自定义的POST或GET方式请求，以及监视不同的HTTP状态码，自己看代码琢磨吧 :)
完整代码：
代码如下：

AjaxObj = function(url, method, content){
    this.r = null;
    this.url = url;
    this.method = method;
    this.content = content;
    this.header = {};
    this.header["Connection"] = "close";
    this.header["Content-type"] = "application/x-www-form-urlencoded";
    var self = this;
    if(window.XMLHttpRequest){
        this.r = new XMLHttpRequest();
    }else if(window.ActiveXObject){
        try {
            this.r = new ActiveXObject("Msxml2.XMLHTTP");
        } catch(e) {
            try{
                this.r = new ActiveXObject("Microsoft.XMLHTTP");
            } catch(e) {
            }
        }
    }
    this.addListener = function(http_status, func){
        if(!this.L)
            this.L=[];
        this.L[http_status] = func;
        return this;
    };
    this.setHeader = function(name, value){
        this.header[name] = value;
        this.r.setRequestHeader(name, value);
        return this;
    };
    this.send = function(){
        if(this.method != "post" && this.method != "get")
            this.method = "get";
        this.r.open(this.method, this.url, true);
        for(var h in this.header) {
            this.r.setRequestHeader(h, this.header[h]);
        }
        this.r.send(this.content);
    };
    if(this.r) this.r.onreadystatechange = function(){
        if(self.r.readyState == 4 && self.L[self.r.status] != null)
            self.L[self.r.status](self.r.responseText);
    };
};