Quassel IRC is described as 'Modern, cross-platform, distributed IRC client based on the Qt5 framework. Distributed means that one (or multiple) client(s) can attach to and detach from a central core that stays permanently online -- much like the popular combination of screen and a' and is a popular IRC Client in the social & communications category. There are more than 50 alternatives to Quassel IRC for a variety of platforms, including Windows, Linux, Mac, Web-based and Self-Hosted apps. The best Quassel IRC alternative is HexChat, which is both free and Open Source. Other great apps like Quassel IRC are WeeChat, Trillian, mIRC and irssi.
This is one of the few listed here that is a near perfect match to Quassel. A bouncer like server is a core component/feature of smuxi. Most of this long list are IRC clients only.