I used to use netsend all the time at college until i got caught sending it to the main server and it sent a message saying "Hello world!" to every computer in college.

The reason i got caught was because it had my computer number on it, but i have recently found the little app that lets you send anonymous netsend messages which i now use and thought you might be interested.

This is the download link:

To use it you either enter a computer number or IP address.