Pidgin is a cross platform instant messaging client that has multiple chat protocols including IRC. It has support for almost every other chat service available.
Discord is a free-to-use VoIP and instant messaging app targeted primarily at gaming communities.
Kopete is a multi-protocol instant messaging client for personal and enterprise use.
Empathy is the default Gnome Shell instant messaging client that supports text, voice, video chat, and file transfers over many different protocols.