Pidgin is a cross platform instant messaging client that has multiple chat protocols including IRC. It has support for almost every other chat service available.
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Pro Support for a wide range of additional chat services
Pidgin not only supports IRC, it also supports plenty of other services such as AIM, Google Talk, MSN, ICQ and plenty of others. This way, you can have all of your conversations in the same app and won't have to deal with multiple chat windows being open.
Pro Free and open source
Pidgin and its code is completely free and open source. Pidgin has no ads and no features behind a paywall.
Pro Highly customisable with plugins
Pidgin not only has a large plugin library built-in, but also an array of third party plugins for a variety of functions.
Pro Convenient notification features
Like other quality IRC clients, Pidgin features notifications that the user can set in order to have a variety of things happen when the users is mentioned, such as an audio cue or changing the system tray icon.
Pro Cross platform
The Pidgin instant messaging client is cross platform, working on Linux, Windows and OSX.
Con No native VoIP support
While there are plugins there is no built in support for VoIP in Pidgin which is time consuming to implement instead of being baked in from the jump.