Is there a LAN chat app?

It has been bugging me (especially post-Snowden) that we use the internet for everything, even if the person we interact with is in the next room.

I recently found Dukto to send files from one computer to another in the local network.

I was wondering if there is a (OS independent) chat application for LAN. It would be so cool to have all people in the same network in a bar/cafe/uni to communicate with another (yes, you'd need a block function (based on MAC-address?)).

Does something like this exist?