KVIrc is a free portable IRC client.
Ranked in these QuestionsQuestion Ranking
Pro Plenty of customization options
Pro Extensive documentation available
Help files and online resources show all of the feature and scripting capabilities.
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.
KVIrc does not need to be installed. It can be kept on a flash drive and run from there.