解决jQuery ajax跨域问题，Google、IE、Firefox亲测有效

直接上最后的结果吧

JS：

 $.ajax({
             type: "GET",
             async: false,
             crossDomain: true,
             url: "www.test.com/TestHandler.ashx",
             data: { Id: "1"},
             dataType: "jsonp",
             jsonp: "callback",
             jsonpCallback: "callbackHandler",
             success: function (data) {
                 //success
             },
             error: function (x, status, error) {
                 //error
             }
         });

服务端（这里我用的是ashx一般处理程序）：

 public void ProcessRequest(HttpContext context)
         {
             JavaScriptSerializer jsonHelper = new JavaScriptSerializer();

             HttpContext.Current.Response.ContentType = "application/json";
             HttpContext.Current.Response.AddHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS");
             HttpContext.Current.Response.AddHeader("Access-Control-Allow-Headers", "Origin, Content-Type,Authorization, Accept, X-Requested-With");
             HttpContext.Current.Response.AddHeader("Access-Control-Allow-Origin", "*");

             var list=数据;

             if (string.IsNullOrEmpty(context.Request["callback"]))
             {
                 context.Response.Write(jsonHelper.Serialize(list));
             }
             else
             {
                 context.Response.Write(string.Format("{0}({1})", context.Request["callback"], jsonHelper.Serialize(list)));
             }
         }