I was looking for a over-the-LAN instant messaging app on Ubuntu to use at home. A port of good old ipmsg is available but the interface is too dated. After trying a few more options I found iptux. It has a polished interface, the chat window is modern and can be minimized to the system tray. It supports both chat and file transfers. A good way to communicate if you have several users on the same wifi. Available in Synaptic.
To show the iptux icon in the system tray I added it in the systray whitelist:
# gsettings set com.canonical.Unity.Panel systray-whitelist "['Pidgin', 'Dropbox', 'qbittorrent', 'iptux']"
To share files over LAN as well as the internet try BaShare.