From XKCD Wiki

List of channels

Channel	#	Description
#xkcd	350	For the comic! Oh, and politics.
#xkcd-signal	180	Robot 9000 experiment.
#meta-discussion	25	For discussion of #xkcd-signal.
#xkcd-crypt	Ø	A strangely encrypted form of #xkcd-signal
#crypt-discussion		For discussion of #xkcd-crypt.
#xkcd-science	2	Science! It works, bitches! And THIS IS WHERE WE DISCUSS IT.
#xkcd-voice		For users of XKCD teamspeak.
#xkcd-rhymers	15	Herein, one may only speak in rhyming couplets.
#xkcd-mime	5	As an antithesis to -rhyme, here one may only speak in actions (i.e. /me)
#xkcd-lolspeak	5	We can has channel where ceiling cat watches you type.
#meta-lolspeak		For discussion of #xkcd-lolspeak.
#xkcd-tv	Ø	For discussion of television.
#xkcd-love	35	For relationship advice and such.
#xkcd-cats	2	See #xkcd-dogs
#xkcd-cs	15	Computer science topics. Redundant with #xkcd-compsci.
#xkcd-compsci	22	Computer science topics. Redundant with #xkcd-cs.
#xkcd-dnd	5	Official D&D channel for xkcd people. We like 3.5e because we're cool like that.
#xkcd-dogs	2	See #xkcd-cats
#xkcd-drug	5	Discussion of pharmacology, including licit as well as illicit drugs.
#xkcd-flu	10	One stop shop for all of your Swine Flu/other disease needs. No pigs allowed.
#xkcd-gtfo	2	I dunno, but if you come over, i'm sure we'll find something to talk about
#xkcd-jobless		These poor people have no employment. :(
#xkcd-language	5	Linguistics, language learning, and artificial languages.
#xkcd-math	ω	MATH, guys. This channel is about MATH!!!
#xkcd-music	10	Music, from theory to production.
#xkcd-noise	2	Everyone's an op, spamming encouraged.
#xkcd-pedophilia	5	We talk about feet. A lot.
#xkcd-politics	5	Bloody politics. They go here, not #xkcd
#xkcd-secret	0	it's a secret
#xkcd-shower	10	This is where dirty goes.
#xkcd-wii	6	Coordination channel for playing Nintendo WFC games.
#xkcdfurs	15	Yes, a channel full of furs. What were you expecting?
#xkcdwiki	5	For discussion of this wiki.
#<3thunder	∞	♥☈
#anarcho-communism	10	It's a Party!
#ART	2	People who draw stuff, whether or not they should, should come here and share ART.
#bots	35	A playground for IRC bots.
#bucket	20	Bucket's private playground.
#geohashing	55	All things related to geohashing
#kittenbbq	25	Delicious kittens.
#mafia	30	Bot-modded mafia game
#uk	2	Uranium Kingdom!
#us	5	join us! It's like notpron or replats-eht. Except it's defunct!
#xchat		For users of the XChat IRC client.
#srd		For the more general discussion of RPGs.

The number of lurkers in a given channel is, of course, unstable. The # given is merely an estimate to aid one's lurking heuristics.