NetDispatch is a freeware app, designed to send single, non-replayable, messages to many Mac computers at once.
Simply setup NetDispatch on any computer you want to receive messages on, and let it run. ND will run completely invisibly, in the background. Once running, it will check for new messages, at all of the specified URL's, and at the desired interval. To send a message, simply change the content of the file at one of the specified URL's to your message, then save. Your message will instantly show up on any computer setup with NetDispatch.
Message files can easily be stored on your local network, using Mac OS X's built in web sever. Alternatively, if you wish to send messages to computers anywhere in the world, your message file can be stored on any web sever or even a Dropbox public folder.