Three I really care about: Advogato, Lambda the Ultimate and the new Tex Stack Exchange.
Stack Overflow isn't a community anymore, but I still go there a few times a week and see what's happened on the tags I care about. I've got a Linked In account: the forums there are weird; the German-centric Xing is better, but not hugely - the advantage of these is that they are easy to keep up with. I checkout Slashdot every month or so, and am not really sure why I bother.
I subscribe to a few mailing lists, but I spend much less time with them than I did six or seven years ago. They feel more a like an obligation that a community, to be honest. I'm a bit bothered about that, since mailing lists are much easier to archive than anything else: internet communities are becoming more transitory in so many ways.