Quick start

Download script, edit its bottom part after words “PUT YOUR CODE HERE”, this one:

(function () {

...

    /* -------------------- PUT YOUR CODE HERE ------------------ */
    loadJquery({
        url: 'http://code.jquery.com/jquery-latest.min.js',
        timeout: 5000, //ms
        success: function ($) {
            $('h1').css('border', '2px solid red');
        },
        error: function () {
            alert("Can't load jQuery.");
        }
    });
}());

Advantages:

• based on jquery 1.7.2, so should be stable and cross-browser
• exceptions handled with help of try/catch
• ability to set download timeout
• jQuery will not be loaded, if found in page
• simplicity for end-user: just write your code in success/error callbacks
• no conflicts with other javascript libraries like prototype.js
• script passes jslint-validation

Now bit more information for those who interested.
Script consists of two parts: loadScript и loadJquery.

loadScript

The function almost completely based on jQuery 1.7.2, which is latest at the moment.
External script is loading by putting SCRIPT tag into HEAD.
Exceptions are handled with try/catch, in theory javascript-erorrs, if any, should be catched without placing error in browser console.
There is possibility to setup download timeout. If timeout reached, download will be stopped and error-callback will be called.

Example of using:

loadScript({
    url: 'http://example.com/script.js', //script url
    timeout: 5000, //optional timeout in ms
    success: function () {
        alert('Script loaded callback.');
    },
    error: function () {
        alert('Script was not loaded.');
    }
});

loadJquery

Few details about this function:

• jQuery will not be loaded, if found in page
• .noConflict() used, so end-user should not worry about conflicts with other javascript library like prototype.js

Full listing of the script.

/*!
 *
 * loadJquery.js
 * Load jQuery on demand from javascript
 * Author: contact@slicezilla.com
 * November 2011
 *
 * The script uses part of jQuery JavaScript Library v1.7
 * http://jquery.com/
 * Copyright 2011, John Resig
 * Dual licensed under the MIT or GPL Version 2 licenses.
 * http://jquery.org/license
 *
*/
/*global document, setTimeout, clearTimeout, alert, jQuery*/
(function () {
    "use strict";
    var loadScript = function (options) {
        var script, transportTimeout,
            head = document.head || document.getElementsByTagName('head')[0] || document.documentElement,
            transport = {
                send: function (url, successCallback) {

                    script = document.createElement('script');

                    script.async = 'async';
                    script.src = url;

                    // Attach handlers for all browsers
                    script.onload = script.onreadystatechange = function (event, isAbort) {
                        if (isAbort || !script.readyState || /loaded|complete/.test(script.readyState)) {

                            // Handle memory leak in IE
                            script.onload = script.onreadystatechange = null;

                            // Remove the script
                            if (head && script.parentNode) {
                                head.removeChild(script);
                            }

                            // Dereference the script
                            script = undefined;

                            //callback if not abort
                            if (!isAbort && typeof successCallback === 'function') {
                                clearTimeout(transportTimeout);
                                successCallback();
                            }
                        }
                    };
                    // Use insertBefore instead of appendChild  to circumvent an IE6 bug.
                    // This arises when a base node is used (#2709 and #4378).
                    head.insertBefore(script, head.firstChild);
                    //head.appendChild(script);
                },

                abort: function (errorCallback) {
                    clearTimeout(transportTimeout);
                    if (script) {
                        script.onload(0, 1);
                    }
                    if (typeof errorCallback === 'function') {
                        errorCallback();
                    }
                }
            };

        //init timeout
        if (options.timeout > 0) {
            transportTimeout = setTimeout(function () {
                transport.abort(options.error);
            }, options.timeout);
        }

        //get script
        try {
            transport.send(options.url, options.success);
        } catch (e) {
            transport.abort(options.error);
        }
    };

    //function which will load jQuery using loadScript
    function loadJquery(options) {
        if (typeof jQuery !== 'undefined') {
            //jQuery is loaded already, just perform business logic
            if (typeof options.success === 'function') {
                //pass jQuery as parameter, in order to have $ === jQuery in callback
                //just in case that $.noConflict was used already, and $ !== jQuery
                jQuery(options.success);
            }
        } else {
            //jQuery was not loaded on a page
            loadScript({
                url: options.url,
                timeout: options.timeout,
                success: function () {
                    jQuery.noConflict();
                    if (typeof options.success === 'function') {
                        jQuery(options.success);
                    }
                },
                error: options.error
            });
        }
    }

    /* -------------------- PUT YOUR CODE HERE ------------------ */
    loadJquery({
        url: 'http://code.jquery.com/jquery-latest.min.js',
        timeout: 5000, //ms
        success: function ($) {
            $('h1').css('border', '2px solid red');
        },
        error: function () {
            alert("Can't load jQuery.");
        }
    });

}());