dxirc is a easy-to-use yet capable cross-platform IRC client. Among its notable features are flexible configuration, a fully customizable look independent of system-wide settings, multiple server support with optional automated joining of selected channels, and the ability to connect via SSL.
Nebula: A small and lean IRC client written for motif/lesstif. Small system requirements, speed, portability, security and a clean GUI are its strong points.
TalkSoup is an IRC client for OS X and any platform supported by GNUstep. It is highly configurable and extensible with a fully-featured plugin system already in place. It has all the features one would expect from a graphical IRC client.