XChat is a popular Internet Relay Chat (IRC) client. It has a choice of a tabbed document interface or tree interface, support for multiple servers and is highly configurable.

XChat is a full-featured IRC client and includes all basic functionalities that are found in most other IRC clients, including CTCP, DCC file transfers and chats.
It supports a plugin system for various languages (including C, C++, Perl, Python, Tcl, Ruby, Lua, CLISP, D, and DMDScript). By writing plugins, the features and functionality of XChat can be extended and customized.