KVIrc is a free portable IRC client.
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Pro Still active developement
Despite whats been said, development has been continuing at a steady pace on version 4.3.1 and more recently 4.3.2.
Pro Plenty of customization options
KVIrc does not need to be installed. It can be kept on a flash drive and run from there.
Pro Extensive documentation available
Help files and online resources show all of the feature and scripting capabilities.