Quassel IRC is a modern, cross-platform, distributed IRC client, meaning that one (or multiple) client(s) can attach to and detach from a central core -- much like the popular combination of screen and a text-based IRC client such as WeeChat, but graphical.
shroudBNC is a modular IRC proxy written in C++. It is capable of proxying IRC connections for multiple users. Using TCL scripts it can be extended.
Amongst its features you can find a user-friendly web interface (coded in TCL and PHP) which allows you to easily create and manage users. A live
... [More] demonstration of the web interface can be viewed here. Some screenshots are also available.
A variety of operating systems are currently supported: Linux, FreeBSD (and possibly other variations of BSD) and Windows. [Less]