$.ajax({
  type: "POST",
  url: "/game-checkin",
  data: dataString,
  beforeSend: function () {
    // ... your initialization code here (so show loader) ...
  },
  complete: function () {
    // ... your finalization code here (hide loader) ...
  },
  success: function (badges) {
    $('#checkin-form').html("<div id='message'></div><div id='badges'></div>");
    $('#message').html("<h2><img class=\"check-mark\" src=\"/static/images/check-mark.png\"/>You are checked in!</h2>");
    $.each(badges, function (i, badge) {
      $('#badges').append("<h2>New Badge!</h2><p><img class='badge' src='" + badge.image_url + "'><span class='badge-title'>" + badge.name + "</span></p>");
    })
  }
});

http://api.jquery.com/jQuery.ajax/:

Here are the callback hooks provided by $.ajax():

beforeSend callback is invoked; it receives the jqXHR object and the settings map as parameters. error callbacks are invoked, in the order they are registered, if the request fails. They receive the jqXHR, a string indicating the error type, and an exception object if applicable. Some built-in errors will provide a string as the exception object: "abort", "timeout", "No Transport". dataFilter callback is invoked immediately upon successful receipt of response data. It receives the returned data and the value of dataType, and must return the (possibly altered) data to pass on to success. success callbacks are then invoked, in the order they are registered, if the request succeeds. They receive the returned data, a string containing the success code, and the jqXHR object. complete callbacks fire, in the order they are registered, when the request finishes, whether in failure or success. They receive the jqXHR object, as well as a string containing the success or error code.

Note the beforeSend and complete method additions to the code.

Hope that helps.

If you're using jQuery 1.5 you could do that nicely, unobtrusively and generically with a prefilter. Let's make a very simple plugin for this:

(function($) {

    var animations = {};

    $.ajaxPrefilter(function( options, _, jqXHR ) {
        var animation = options.animation && animations[ options.animation ];
        if ( animation ) {
            animation.start();
            jqXHR.then( animation.stop, animation.stop );
        }
    });

    $.ajaxAnimation = function( name, object ) {
        if ( object ) {
            animations[ name ] = object;
        }
        return animations[ name ];
    };

})( jQuery );

You install an animation as follows:

jQuery.ajaxAnimation( "spinner" , {
    start: function() {
        // code that starts the animation
    }
    stop: function() {
        // code that stops the animation
    }
} );

then, you specify the animation in your ajax options:

jQuery.ajax({
    type: "POST",
    url: "/game-checkin",
    data: dataString,
    animation: "spinner",
    success: function() {
        // your success code here
    }
});

and the prefilter will ensure the "spinner" animation is started and stopped when needed.

Of course, that way, you can have alternative animations installed and select the one you need per request. You can even set a default animation for all requests using ajaxSetup:

jQuery.ajaxSetup({
    animation: "spinner"
});

you can set global ajax loading icon handler using here #ajxLoader takes your loading icon

   $( document ).ajaxStart(function() {
        $("#ajxLoader").fadeIn();
    });

    $( document ).ajaxComplete(function() {
        $("#ajxLoader").fadeOut();
    });