Recs.
Updated
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.
SpecsUpdate
Pros
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.
Cons
Con Rapidly diminishing applications
It supports AIM! Awesome, except AIM is gone. Google Talk works but it's called Hangouts now and doesn't and no wait now it doesn't even exist. ICQ works but might be a Putin blackmail-collection scam, MSN Messenger long ago turned to Skype which might die in 2022 and...
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.
Con Miserable IRC font controls (as of 2.13.0.17)
Does its best to avoid putting sensible default font controls in any place where you can get to it. And when it does grudgingly change it, based on system preferences (not local preferences), it does so in an inconsistent and unpredictable fashion.
Recommendations
Comments
Flagged Pros + Cons
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.
Out of Date Pros + Cons
Con Pidgin does not support yahoo.
Pidgin no longer has options to support yahoo messenger. Most likely due to yahoo being sold to verizon. The vast majority of pidgin users were also yahoo users, and so pidgins usability has declined.