#hn is, well, HN. But nobody ever really seems to discuss anything, so most of the traffic is just a bot that posts new links that show up on HN. Still, no reason in principle people couldn't discuss things there. Possibly they would if more people were in the channel?

#startups - populated by a lot of HN users, but the discussion is pretty random, and not necessarily limited to (or even often) about things you see on HN.

##machinelearning - fairly low volume, but pretty focused on ML

#ml-ot - for off-topic discussion / idle chit-chat, vaguely related to machine learning.

#ai - very low volume, but good every now and again, in spurts

##agi - even lower volume, so please join and contribute if you're interested in Artificial General Intelligence!

#cyberpunk - cyberpunks

#postcyberpunk - postcyberpunk

#tensorflow - tensorflow

#wikidata

#swig - Semantic Web Interest Group

##cypherpunk

etc.