Like one that would work without internet access?
Across the network?
It depends on the network, there is some software for pcs that i've used but most networks block the ports that the messages are sent across specially in school networks..
http://www.google.com.au/search?num...&hs=NzK&q=network+messenger&btnG=Search&meta=
Theres tonnes there, but the first two hits look pretty good.