Notificant delivers Growl-like notifications across multiple Macs
Sure, with a little bit of tinkering in the Terminal, Growl can actually send notifications to other computers on your local network, but that doesn't help when you're out and about with your MacBook. With Notificant, you can quickly whip up a new reminder on your desktop machine using a Web interface, and have that reminder go off later on your laptop. It's like setting iCal reminders without iCal, and it works on as many computers as you want.
Notificant will run you 5 bucks in the Mac App Store, and you'll have to set up a (free) account to start using it.