Windows

Back in the days of Win95 there was a little utility called 'WinPopup' which would send popup message onto another networked computer's screen.

It appears this has been revamped and still works on modern computers:

http://www.freedownloadscenter.com/Search/winpopup.html

It needs to be installed on each computer.

I haven't tried it out but it should work alright. :)

control panel > add & remove programs > windows component tab> check the box for terminal services > ok then restart.
also ridgth click My comp.>>Properties>>Remotecheck mark the 2 checkbox.

U can use terminal services to connect to a remote client and on right clicking the client name there you get a send message option

For details on TS u can view this page

Just copy and paste the above link in your browser
http://www.governmentsecurity.org/forum/index.php?showtopic=10869

Use IP Messenger. It is free to download. You can even send files.
It is available at http://www.ipmsg.org/index.html.en