net send is a messaging utility you can access using command prompt. It sends a message to whatever computer/username you specify.
For example
code:
net send madman66 "Im am sending a message"
Would send the user "madman6" the message "Im am sending a message".
And it would popup on his screen in a dialogbox that shows the computer that the senders on, and the message.
This can be disabled though and some schools have disabled it.
Oh and another hint for net send: Wildcards
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