Well, the big ones at least. I recently “discovered” you can create arbitrary rooms on Freenode. Given how much I love Automattic’s IRC backchannels, I decided to create a few rooms for the plugins I work on on the off-chance they make it easier to answer quick questions, discuss feature requests, etc.
If you like #editflow, #coauthorsplus, #adcodemanager, or #supportflow, feel free to join me.