iOS/Windows/Mac/Linux: Free service Notifo is a kitchen sink of push notifications for your iPhone or desktop, pushing Twitter updates, your favourite RSS feeds, Google Voice alerts, and more straight to your iPhone or desktop notification utility. And it's extremely hackable.
Notifo is pretty similar to previously mentioned Boxcar (inasmuch as it makes it simple to set up push notifications for a lot of popular services), but the big difference is that it's very hackable. So in addition to supporting the popular services mentioned above, Notifo can also send notifications from your desktop to your iPhone (supporting Growl and Growl for Windows), from the command line to your iPhone (great for command line scripters), from Chrome to your iPhone (see the Chrome to Notifo extension), and a whole lot more.
On top of all of that, Notifo also has a built-in messaging app that allows you to chat back and forth with other users who've installed Notifo—with lightning-fast push notifications between devices.
In short, Notifo is a great little app with all sorts of potential. My only criticism is that it's a a touch amorphous; I'm not quite sure if it's a powerful notification app or a messaging tool (it's both, but the connection between the two is a little tenuous). Still, the app's developed by a very smart friend of Lifehacker, Paul Stamatiou, and we can only expect that it'll get better and better.
Notifo is a free download for iOS.