if (typeof jQuery == 'undefined') {  
    <script type="text/javascript" src="http://www.google.com/jsapi"></script>
    <script type="text/javascript">
    google.load("jquery", "1.4.2");
    </script>
}

http://www.plan-zero.org

If you are working on apps development to adapt on different sites that you don't have access to them, and need to load Jquery, but don't wanna load it twice this will help you !

Check if Jquery Exist if not load it

ajax, google, jquery, load

jQuery