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javascript:(function () {
    var wasUndefined = typeof jQuery == 'undefined';
    var download = prompt('Inject jQuery <version> as global <name>:', '1.7.2 ' + (wasUndefined ? 'jQuery' : 'ae'));
    if (! download)
    {
        return;
    }
    download = download.split(' ');
    var newVersion = download[0];
    var newJQuery = download[1];
    window.oldJQuery = wasUndefined ? null : jQuery;
    window.jQuery = null;
    var s = document.createElement('script');
    s.setAttribute('src', 'http://ajax.googleapis.com/ajax/libs/jquery/' + newVersion + '/jquery.min.js');  
    document.getElementsByTagName('head')[0].appendChild(s);  
    
    var time = 0;  
    var id = setInterval(function () { 
        var isUndefined = typeof jQuery == 'undefined'; 
        if (! isUndefined) {  
            clearInterval(id);  
            jQuery.noConflict();
            console.info('jQuery ' + jQuery.fn.jquery + ' has been injected. (as ' + newJQuery + ')');
            window[newJQuery] = jQuery;
            window.jQuery = wasUndefined ? jQuery : window.oldJQuery;
            return; 
        }  
        ++time; 
        if (time == 10) {  
            clearInterval(id);  
            console.info('Sorry, unable to inject jQuery now.');
            window.jQuery = window.oldJQuery;
        }  
    }, 500); 
})();

UPDATED: 2012-06-10