XChat

XChat is a popular Internet Relay Chat (IRC) client. It is available for Unix-like systems, Microsoft Windows (with additional features), Mac OS X (X-Chat Aqua), and for other X Window System-based systems (Fink project). It has a choice of a tabbed document interface or tree interface, support for multiple servers and is highly configurable. Both command line and graphical versions are available. The main code is licensed under the GNU General Public License, but the official Windows version is shareware, and uses the GTK+ toolkit for its interface. It is also available as an app for Joli OS.

Read more about XChat:  Features, Reception