Edit in JSFiddle

var SpriteAnimation = new Class({ // MooTools Class ...

    Implements: [Options, Loop], // looks normal so far
 
        options: {
                frameWidth: 75,
                frames: 10,
                frameRate: 100,
                defaultPosition: {x: 0, y :0}
            },
    
    jQuery: 'animateSprite', // huh?

    initialize: function(element, options){
        this.setOptions(options);
        this.setLoop(this.step, this.options.frameRate);
        this.element = jQuery(element); // jQuery?!
        var pos = this.options.defaultPosition.x + 'px ' + this.options.defaultPosition.y + 'px';
        this.element.css({'background-position': pos}); // moar jQuery!?
        this.startLoop();
    },

    step: function(){
        var x = this.computeX();
        var y = this.computeY();
        this.element.css({'background-position': x+'px '+ y+'px'}); // it's taking over!
        return this;
    },
    
    computeX: function(){
        this.loopCount = (this.loopCount == (this.options.frames)) ? this.options.defaultPosition.x : this.loopCount
        return -this.loopCount * this.options.frameWidth;
    },
    
    computeY: function(){
        return this.options.defaultPosition.y;
    }

});

// what on earth is happening?!
jQuery(document).ready(function(){
    
    // instantiate
    $('#spinner').animateSprite({
        frameWidth: 64,
        frames: 12,
        frameRate: 50
    });
    
    // or the object oriented way ...
    /*
    var spinner = new SpriteAnimation('#spinner',{
        frameWidth: 64,
        frames: 12,
        frameRate: 50
    });
    */
    
    $('#stop').click(function(){
        
        // call methods on it w/ the jQuery object.
        $('#spinner').animateSprite('stopLoop');
        
        // or the object oriented way ...
        // spinner.stopLoop();
        
        $('#spinner').fadeOut();
        $('#start').attr('disabled', false);
        $(this).attr('disabled', true);
    });
    
    $('#start').click(function(){
        $('#spinner').animateSprite('startLoop');
        $('#spinner').fadeIn();
        $('#stop').attr('disabled', false);
        $(this).attr('disabled', true);        
    });
    
});

#ftw {
    font-family: helvetica, arial;
    position: relative;
    background: #ccc;
    color: #000;
    padding: 20px;
    font-size: 20px;
    
    -webkit-border-radius: 0.5em;
    -moz-border-radius: 0.5em;
    -webkit-box-shadow: 0px 3px 5px rgba(0,0,0,0.5);
    -moz-box-shadow: 0px 3px 5px rgba(0,0,0,0.5);
    text-shadow: #fff 0 1px 0;
}

#spinner {
    width: 64px;
    height: 64px;
    background-image: url(http://ryanflorence.com/jsfiddle/mootools-jquery/spinner.png);
    background-repeat: no-repeat;
    background-position: 0 0;
    position: absolute;
    top: 24px;
    left: 20px;
}

<h1 id="ftw">
    I'm a MooTools Class,<br/> 
    but I use jQuery for everything else.<br/>
    Alpha transparency is cool.
    <div id="spinner"></div>
    <br /><br />
    <button id="stop">stop</button>
    <button id="start" disabled="true">start</button>

</h1>