Quassel is a modern, cross-platform, distributed IRC client.
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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 Free and open source
Quassal is available for free with source code licensed under GPL and available here.
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.