Windows messaging is a service that runs on the background of Windows (this is the default for Windows up to XP SP2. After that you have to manually turn this feature on). It's purpose is to send and receive messages to and from other users on the same local network (on older versions of Windows it used to accept messages from the Internet too, which was a welcoming gesture for spammers.). When you receive a message a window pops up containing the name of the computer that sent the message, the time on which it was sent and the message itself. Messages can be send by issuing the NET SEND user message command on the command line. But this of course will reveal the name of your computer to the recipient.
Anonymous NET SEND allows you to spoof your computers name while sending a message to another user.
You can download Anonymous NET SEND by clicking here.
The source code can be found here. (VB6 source released under GNU/GPL v3 or later)
Here is a screenshot: