The Linux operating system has gone through a number of “default” chat applications. From the earlier text-based chats like zicq, to the later Gaim, then to Pidgen (still in use), and now (and, hopefully, finally) GNOME has settled on Empathy.
Empathy is a messenging program built upon the Telepathy protocol which is based on Gossip. With the help of Telepathy, Empathy can easily communicate with other GNOME applications, for a seamless, integrated desktop experience. Empathy supports: Multiple protocols, file transfer via XMPP and local networks, voice and video support, limited IRC support, conversation theming, sharing of location, conversation logging, and much more.