Empathy is very small app, integrated with Gnome and login is not required. Try feature named people nearby
[With people nearby] you do not need to connect and authenticate to a central server in order to use it.
This kind of serverless messaging system is restricted to a local area network and an active Internet connection is not necessary.
The people that use this service inside the same local area network will be auto-discovered, and it will be possible to send them messages and files as with other services.
Empathy is best choice when your Linux distro is using Gnome as default (empathy is default gnome client).
Read how to Register a new account for LAN communication:
This service works whenever you are connected to a local network, such as a wireless hotspot, you do not need to create an account with service provider, just set up your account through Empathy. It automatically finds all other users on the network who are also using this service.