AthenaIRC is a free, Aqua savvy IRC client designed for all those people who want a simple and efficient client. Its small footprint doesn't take up a lot of desktop space, while still offering a lot of conveniences. Some features of note include: Channel History, Channel Favorites, Command Aliases, Channel Taskbar, Nickname completion, User defined contextual menu pop-ups, optional single window interface, buddy list style notify system.
Note: Using the main connection system is freeware and fully functional. MiniConnection windows, which allow multiple connections, are a shareware feature and are limited to one channel at a time until registered.