savIRC is an IRC client for Linux, Unix, BSD, and Windows. It supports multiple server connections, SOCKS5 proxy, CTCP, DCC, passive DCC, ignore and notify lists, Unicode, mIRC colors, MDI dedicated windows, logs, customizable popups, pre-defined events, scripting in Tcl/Tk, captions for URL nicks, and channel names.
TiK is a pure TCL/Tk implementation of AOL Instant Messenger (AIM). It has most of the features the native Win32 and Mac clients have, plus some extras. It includes a graphical buddy list display, full font support, graphical emoticons, away messages, buddy pounce, buddy ticker, IM/Chat capturing, message broadcasting, a GUI preferences system, and much more. TiK's expandable nature allows the addition of new features through packages.
OpenVerse is similar to other graphical chat systems like Excite Chat and Microsoft Chat, with the main differences being that OpenVerse is open source software, has no advertisements, and also runs on a variety of systems including Windows, Unix, Macintosh, and just about anything else for which you can find a copy of TCL/TK.