netsend

It's a little command line program included with windows that you can set up to send short messages to computers as a popup box. A lot of printers could use this to tell you your print job was successful, and it was used a lot in libraries and such. And also my high school. They had some cursory protections in place, but if you managed to open a command prompt you could send your own message. You just needed the recipients windows username or PC name.. our school used the standard first letter of first name + full last name, even the teachers. So of course, being highschool, this spread like wildfire and there was a whole semester where everyone was abusing it to troll other classmates or interrupt teachers mid lesson. It was also being used as IM/text before any of us even had phones - you could shoot your friend a message to dip out of class or something.

Everything came to an abrupt halt when a guy was dared to run a batch file that was a single, looped, expletive laden net send to a wildcard recipient. It sent the message on repeat to every computer in every school in the district. Every time you hit ok a new box would pop up with the same message. Supposedly every computer needed a hard restart, including servers. Dude got in trouble, and our printers stopped telling us the print job was successful after that.