KVIrc (K Visual IRC) is a customizable Internet Relay Chat client allowing multiple server connections, notify and ignore lists, mIRC font codes, themes, and more. The program supports standard + extended direct connect connections (with resume), IPv6, SOCKv4, SOCKSv5, and HTTP proxies. It also comes with a sophisticated scripting language that allows you to implement automated reactions to network events.
The program is cross-platform with clients available for Mac, BSD and Linux. KVIrc Portable is a portable wrapper for KVIrc. Alternatively, you can make standard KVIrc portable by following these instructions.