consume a asp.net web service that returns json from an stand alone html page

<asp:ScriptManager ID="ScriptManager" runat="server">
	<Services>
		<asp:ServiceReference Path="~MyService.asmx" />
	</Services>
</asp:ScriptManager>

$.ajax({
    type: "POST",
    url: "~/MyService.asmx/MyMethod",
    data: "{parameterName:'" aStringArgument + "'}",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function(msg) {
       var data = msg.d
       // Now var is an object with properties just like your object
    }
  });

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public String Foo(String p1, String p2)
{
    return "Hello World";
}

var httpobj = getXmlHttpRequestObject();
//Gets the browser specific XmlHttpRequest Object
function getXmlHttpRequestObject()
{
    if (window.XMLHttpRequest)
        return new XMLHttpRequest();
    else if(window.ActiveXObject)
       return new ActiveXObject("Microsoft.XMLHTTP");
}

CallService()
{
    //Set the JSON formatted input params
    var param = "{'p1' : 'value1', 'p2' : 'value2'}";
    //Send it to webservice
    if(httpobj.readyState == 4 || httpobj.readyState == 0)
    {
        httpobj.open("POST", 'service.asmx/' + 'Foo', true);
       //Mark the request as JSON and UTF-8
       httpobj.setRequestHeader('Content-Type','application/json; charset=utf-8');
       httpobj.onreadystatechange = OnSuccess;
       httpobj.send(param);
    }
}

//Called on successfull webservice calls
OnSuccess()
{
   if (httpobj.readyState == 4)
   {
       //Retrieve the JSON return param
       var response = eval("(" + httpobj.responseText + ")");
   }
}