Chatzilla is a full featured IRC client integrated with the Seamonkey Internet Suite. It is scriptable, and supports what most people would expect from an IRC client. Read more
Hircules is an IRC client written in Haskell. The code was initially based on lambdabot and uses gtk2hs for the UI. It has tabs for channels and private dialogs with highlighting. I'm planning to add more features like completions, coloring of nicks and conversations, better keybindings and UI.
LostIRC is a simple, yet very useful IRC client. It has features such as
tab-autocompletion, multiple server support, automatic joining of
servers/channels, logging and DCC sending which should cover the needs of
most people. Another goal that the application has in mind, is 100% keyboard
controlability. It was written using the gtkmm GUI library.
Polari is an Internet Relay Chat (IRC) client designed for GNOME 3.
Smuxi is a cross-platform (using Gnome# and Gtk#) IRC client for advanced users with multiple server support. It's based on the SmartIrc4net library. Read more
XChat is a graphical IRC client. It runs under the X Window System and uses the GTK+ toolkit. Optionally it can be compiled to use Gnome. Read more
xchat-gnome is a GNOME frontend for the popular X-Chat IRC client that was designed with an emphasis on HIG compliance and desktop integration.
yairc is an integrated IRC client for X11 using Perl/Tk or Perl/Gtk. It supports multiserver, DCC, and scripts, and is small and clean.