#startups, #django, #clojure but to tell you the truth it changes all the time. for me it's not really a social thing - i'm just scanning for the occasional nugget of wisdom, or seeking help.
#nmu on csc.nmu.edu My CS department runs an unofficial server that we all SSH into and screen through IRSSI. We get a lot done that way. Makes planning our ACM/LUG meetings easier too since you can scrollback and see the previous discussions.
Also if Colloquy isn't your thing, there's also LimeChat and Linkinus. I've got lots of bitching that Colloquy's a dead project (with commits from a day ago...) and people who decide to switch have found those two clients to be useful, although Linkinus is shareware.
According to searchirc.com QuakeNet peaks at about 170k users, whereas Justin.TV chat has gone up to 188k concurrent users so far.
I should write something about how JTV Chat was implemented - there's some interesting stories.