Quassel IRC is a modern, cross-platform, distributed IRC client.
Quassel has a modern GUI with dockable nicklist and topic bar and customizable views.
Quassel is built on Trolltecs Qt4 libraries. That means that it will run under Linux, MacOS, and Windows.
Quassel optionally supports a stand alone so called core that can stay connected to the irc channels. One ore more Quasselclients can connect to the core and continue chatting where ever they left the network. That includes a kind of BNC functionality so that you can follow the discussions happened during your absence.