Growl is by far the most awesome Mac notification utility there is -
it shows me when stuff happens on my computer, like when Firefox has
finished downloading files, like when Transmit has finished uploading
files, and when stuff finishes zipping, when contacts go online or
offline, and so on. It's pretty awesome.

Enter Prowl.
Prowl is an iPhone app, and it uses pish Notification technology on
the iPhone to send me a notification whenever I'm away from my
computer - essentially, it's like having Growl notifications on my
Mac, sent to my iPhone.

The really cool thing about this is, I can leave a file downloading at
home, and when it's done, Transmission will show a Growl notification on my laptop to let me know that it's done - but since I'm not at the computer, Prowl automatically picks up the notification and pushes it to my device - with no interaction required from me.

Now obviously this kind of notification system could be extended to oter systems - already, people have created extensions and plugins so you can get a Prowl notification whenever someone comments on your Wordpress blog, for example, or when someone mentions you on IRC, or when you get a message/call on Google Voice, or even when you have a new RSS item to be read. Infinitely extensible, and easy as hell to use - all while being pretty damn cool, to boot.

Twilight may have dampened my hope for humanity, but it's stuff like this that makes technology so awesome.