A while ago I “released” limegreen, an application template for making java applications look as good as they can, and function as natively as they can, on OS X, windows and linux.
Anyway, I use it in some of my applications, and I’ve recently added a completely automatic autoupdate feature. It will produce an ATOM file with the version in it and change history, and links for where to get the update files. I choose ATOM, because it meant with the same file I could store information that could be read by a program that needed to know if it should update, as well as allow people to subscribe and be notified of updates in a feed/RSS reader.
I’ve also released a PHP library and wordpress plugin that allows the ATOM file to be read, and version information to be automatically placed on a website. Its what I use to show this, and generated the latest download links on my website. If you’re looking for the PHP library, just download the wordpress plugin and include limegreen.php.