- You can easily configure Pidgin to work over a LAN using the Bonjour protocol. http://www.ghacks.net/2008/11/19/im-over-a-lan/ From what I understand, they don't build any security into the protocol so you'd obviously want to add one of multiple encryption plugins for already-populated networks (i.e. not just your stuff).
- Another open source LAN chat client with a lot of great features that looks really sharp and claims portability. Negatively, requires dotNET 4.5 for the latest version. https://squiggle.codeplex.com/
- Nitroshare looks great (cross platform, OSS) but the last update was mid-2012 and a lot of links on the homepage were broken. https://launchpad.net/nitroshare
- Bingo's Chat - a feature-rich, secure, serverless local IRC client. Looks pretty amazing but hasn't been developed since 2010 and the homepage is in Russian: http://bingosoft.info/programs/bingos-chat.html
http://www.softpedia.com/get/Internet/C ... Chat.shtml - LuLu Messenger - GPL, Windows-only, looks a bit more like a modern and not stripped-down IM client like many others but again hasn't seen activity since 2012. Supports sending screenshots, pictures, and whole directories.
http://www.softpedia.com/get/Internet/C ... nger.shtml
- Isotoxin (added to the database) allows for LAN-based chat, VoIP and a lot more.