Best non-IM use of iPhone 3.0's push notifications yet: Prowl is a $3 app that pushes Growl notifications from your Mac to your phone. Growl's a global notifier that plugs into everything from BitTorrent apps to iTunes to Mail.
The possibilities really are endless: You can be pinged with a push notification when a torrent finishes downloading, you get a new IM or email, you're mentioned on Twitter, or anything else Growl can notify you about.
You can customize which notifications are pushed to your phone and when they're sent (like only if you're not at your Mac), and Prowl stores up to 30 days of them. Way awesome, since it effectively makes iPhone push notifications infinitely extensible. Update: Ooo, support for Growl for Windows is coming soon too (thanks Samsita!). [iTunes, Prowl via Daring Fireball]