If you ever where in a big LAN network with a little bit older computers (XP or older) you probably know the Windows Messenger Service.
With the command netsend you are able to send messages directly onto the screen on another person. Unfortunately your computername will appear as sender, and everyone will know you send the message.
If you know want to anonymously send messages you can use this little tool which lets you freely choose the displayed sendername, you can even choose the name of another computer and impersonate someone.
I'm sure some of you are able to think about a few use cases for this tool ...