In Smart Home Hacks, Hack #31 Broadcast Messages On Your Home Network, describes how to use the Mac OS X program LanOSD to send animated, overlaid onscreen messages to every computer on your network. For example, you might want to pop-up a message with the latest outdoor temperature, or when the motion detector at your front door has been triggered, as shown below.
LanOSD, which is free, works great for this. There's hardly anything to configure, just run the application and messages sent from any computer will simultaneously appear on the screens of all of the computers on your local network. You can send messages using AppleScript, which is what I do in conjunction with XTension, or via a nifty command line program that's included with the download. It's perfect for integrating with your shell, perl, or python scripts.
I recently came across a similar program, Growl, that also provides beautiful onscreen notifications. It's supported by a (growing) number of applications, but it doesn't broadcast messages on your network. It can still be pretty handy, and perhaps some enterprising developer will create a growl-compatible notifier that allows distributed messaging.