Quassel is a modern, cross-platform, distributed IRC client.
Ranked in these QuestionsQuestion Ranking
Pro Free and open source
Quassal is available for free with source code licensed under GPL and available here.
It's possible for a front-end client or multiple front-end clients to connect to a single core client. This allows setting up an always on-line core that can be accessed from anywhere.
Pro Easy installation on Ubuntu due to the existence of PPA's
Right now Quassel has a PPA maintainer, which allows you to install the stable version or daily versions.
Support for PostgreSQL as database backend and easy to configure for people that know a bit about systems.
Con Potential memory bloat in monolithic client if left running for a period of time
(I don't know if this problem has been fixed in recent years, but I have doubts)
Con No DCC support
DCC or Direct Client-to-Client protocol allows for private communications between users (without the traffic being routed through servers). Neither chat nor file transfers are supported.