Most Mac OS X users are probably familiar with Growl. For everyone else: Growl is a universal, system wide notification system. Applications can use it to notify the user of events, and the user can tell Growl exactly how he/she wishes to be notified. Basically, it’s Windows XP’s popup balloons on steroids. 
Last weekend, I discovered a rather interesting plugin for Growl, named Prowl. What it does is actually quite interesting. It takes notifications sent to Growl, then uses Apple’s push notification service to deliver them right to your iPhone (or iPod touch, or iPad when available). There’s one catch: it requires installation of a $2,99/€2,39 app on your iPhone. Rather steep for a seemingly simple, one-purpose app. Any and all notifications sent through Prowl, though, are absolutely free of charge. And hey, they have to pay for their server bills, too. 
Now here comes the good part. I wrote seemingly simple for a reason. That’s because Prowl has one more feature, and a very impressive one at that. Besides Growl being able to pass it notifications, Prowl also exposes an API that web services can send notifications to. Those will be pushed to your iPhone as well. In other words, Prowl basically provides a general purpose push notification service for iPhone users.
Where Prowl’s original purpose is pretty interesting in itself, this API is downright genius. Think of the possibilities. Want to be notified of comments on your WordPress blog? No problem! Want to be notified of direct messages and/or mentions on Twitter? Prowl’s got you covered!. Want to send notifications from your own web service? Libraries abound! Practically anything is possible.
There’s one more nice little touch to this: the iPhone app lets you open different applications to “view” the push notification, based on the sending application’s name. For instance, you could have a notification of a Twitter mention launch Tweetie for viewing1, instead of Prowl itself.
Now it’s only a shame that Apple won’t guarantee (timely) delivery of any push notification, or this could be a pretty reliable warning system for all kinds of stuff that needs to be monitored. As it is, it’s definitely useful as such, so long as other (more conventional) means of warning are still in place, or if the system being monitored is not mission critical.
1. Which kind of begs the question why Tweetie itself doesn’t have push notifications… 
