Prowl is an iPhone app that forwards Growl notifications from your Mac to your iPhone. That's a good idea in it's own right. But then there's a little note at the bottom of the FAQ that mentions that there's a little Perl script which allows you to push any notifications to it. So it becomes a generic push notification system for anything that you can connect to it. Like Twitter direct messages, or new comments on your blog, or well... anything. And you could rewrite that Perl script in your language of choice. It's just an HTTP GET with Basic Auth. And that makes it pretty awesome.