Yet another lazy script-loader, with callbacks managed using jQuery Deferred objects.

lazy-loader.js

var $LL = (function($) {
    var deferreds = {};
    return function(s, c) {
        if (s in deferreds) {
            deferreds[s].done(function(){
                c();
            });
            return;
        }
        deferreds[s] = $.Deferred();
        var d = deferreds[s].done(function() {
            c();
        });
        var f = function () {
            if (!(this.readyState
                  && this.readyState !== "complete"
                  && this.readyState !== "loaded")) {
                this.onload = this.onreadystatechange = null;
                d.resolve();
            }
        },
        g = document.getElementsByTagName("head")[0],
        i = function (h) {
            var a = document.createElement("script");
            a.async = true;
            a.src = h;
            a.onload = a.onreadystatechange = f;
            g.appendChild(a);
        };
        i(s);
    };
})(jQuery);