Update March 2014: This code is now available on GitHub

There has been a problem with using window.onload in JavaScript. This event handler waits until all the images and other files load before executing. If you need some JavaScript to execute when the page loads, you usually only need the HTML to be downloaded, not all the images.

The event to use for this is "DOMContentLoaded", but only Firefox (and recently, Opera 9) support this. There are different ways to do this in every other browser, and people have been working on finding a complete solution to this for some time.

Very recently, this problem has been solved by Dean Edwards, Matthias Miller and John Resig. There is a unique solution for Internet Explorer, Safari, and for W3C-compatible browsers (Firefox and Opera 9).

Very soon after, Dan Webb adapted the solution to prototype. Unlike the original solution, this code allows you to add more than one function to be executed when the DOM loads.

Inspired by Simon Willison's addLoadEvent function, I wanted to create a standalone generic solution that anyone could use without needing a specific framework.

So here's the js file: adddomloadevent.js, and here's the actual code:

/*
 * (c)2006 Jesse Skinner/Dean Edwards/Matthias Miller/John Resig
 * Special thanks to Dan Webb's domready.js Prototype extension
 * and Simon Willison's addLoadEvent
 *
 * For more info, see:
 * http://www.thefutureoftheweb.com/blog/adddomloadevent
 * http://dean.edwards.name/weblog/2006/06/again/
 * http://www.vivabit.com/bollocks/2006/06/21/a-dom-ready-extension-for-prototype
 * http://simon.incutio.com/archive/2004/05/26/addLoadEvent
 * 
 *
 * To use: call addDOMLoadEvent one or more times with functions, ie:
 *
 *    function something() {
 *       // do something
 *    }
 *    addDOMLoadEvent(something);
 *
 *    addDOMLoadEvent(function() {
 *        // do other stuff
 *    });
 *
 */
 
addDOMLoadEvent = (function(){
    // create event function stack
    var load_events = [],
        load_timer,
        script,
        done,
        exec,
        old_onload,
        init = function () {
            done = true;

            // kill the timer
            clearInterval(load_timer);

            // execute each function in the stack in the order they were added
            while (exec = load_events.shift())
                exec();

            if (script) script.onreadystatechange = '';
        };

    return function (func) {
        // if the init function was already ran, just run this function now and stop
        if (done) return func();

        if (!load_events[0]) {
            // for Mozilla/Opera9
            if (document.addEventListener)
                document.addEventListener("DOMContentLoaded", init, false);

            // for Internet Explorer
            /*@cc_on @*/
            /*@if (@_win32)
                document.write("<script id=__ie_onload defer src=//0><\/scr"+"ipt>");
                script = document.getElementById("__ie_onload");
                script.onreadystatechange = function() {
                    if (this.readyState == "complete")
                        init(); // call the onload handler
                };
            /*@end @*/

            // for Safari
            if (/WebKit/i.test(navigator.userAgent)) { // sniff
                load_timer = setInterval(function() {
                    if (/loaded|complete/.test(document.readyState))
                        init(); // call the onload handler
                }, 10);
            }

            // for other browsers set the window.onload, but also execute the old window.onload
            old_onload = window.onload;
            window.onload = function() {
                init();
                if (old_onload) old_onload();
            };
        }

        load_events.push(func);
    }
})();

Update: I added a demo page, as well as a compressed version (only 876 bytes).

Update: Thanks to Adam Schlag's suggestion, I fixed the memory leak in IE.

Update: Thanks again to Rob Cherny and Alistair Potts, I've updated the solution so that it works over HTTPS without any security issues. I also decreased the compressed size to 761 bytes. Now it's perfect! (I hope!)

Update: I've gone back and reworked the code to get rid of the global variables, and I've managed to reduce the compressed size to 563 bytes!

Update [Aug. 19, 2007]: Now the script preserves any existing window.onload function, and also executes functions instantly when called after the page has already loaded. But now the compressed version is a hefty 617 bytes.