The downside is that you're trusting your Google Voice and Twitter logins to third-party services, but if you wanna get push notifications about @mentions or DMs from Twitter or SMS messages from Google Voice without keeping a computer running at home with Growl whenever you're out, they're the way to go.

Any other services that use Prowl this way? [GVMax, Prey Fetcher, Thanks Zachary!]

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]