I just learned that Pidgin can do serverless, no-configuration LAN instant messaging by using a Bonjour “account”. All you need to do is add an account as normal, select “Bonjour” from the dropdown and add a username. I also had to punch a hole in the firewall of the computers I was using (tcp, check account “advanced” settings tab for port number). Specifically, when one person wanted to initiate a conversation with another, that other person’s firewall needed to be open.