I'm almost always on ##linux ##philosophy ##programming and #music on freenode :) Why I like them? They are about topics that interest me, in #music people often link stuff I'd have no other way of discovering, and if I am getting into a groove, I can link to something and someone else will link something similar, which I may have not heard before.

Then there's the anonradio channel, but also "com" on the sdf, and the #tildetown channel on.. tilde.town :) Those are mainly just to hang out and talk to people about whatever.

Now, your turn, answer your own question! :-)