Not really, not without having Windows loaded on your machine. If you had windows you could use NET SEND command to send a popup message but Mac doesn't have these commands.
Not without messaging software on both machines. I just got done writing software that will do this on a gov network across multiple devices. Sorry, it isn't possible.
You would have to find some sort of messaging software and use it like instant messenger. The alert program that we wrote runs a service in the background that listens for incoming messages and then displays the message on the screen. We wrote it originally for Windows (where I work net send is disabled) and then had to write a mac and solaris module as well.
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