Tumblr. Dear god, Tumblr. It's evil, especially once you start following a lot of constantly-updating folks.
I've also started going to Kongregate and seeing if there's any game that looks interesting on the front page. So now I'm playing Sudoku and I'm not completely sure why...
Here largely, sad to say. (Belatedly) got into Twitter earlier this year but, after a month of it gradually expanding to exclude any other online habits, I managed to go cold turkey. And without ever posting a single tweet; I just read other people's. Ho-hum.
I occasionally go for the odd roam around Second Life under a variety of alts, just to check out what people are creating there. But their recent-ish update to Viewer 2 client UI has put me off - supposedly easier for noobs to use but it sucks compared to the old set-up. Or, more likely, I'm just getting old and (even more) neophobic.
One I've become rather fond of lately is "Kingdom Rush", which can be played for free at Armor Games. However, if you donate a tiny amount of money (even a buck), you unlock a bit of fun premium content which does make the game a little easier.
In addition to TV Tropes and Cracked I often while away some time by perusing the astronomy section of Yahoo Answers.
You've got to be careful with it though - you start off laughing hysterically at the terrible state of modern science education, then transition to anger, and (if you don't pull out in time) end up in a state of deep despair over the future of humanity.
Wikipedia. Great as a starting point for research, with obvious potential to follow link after link and end up nowhere near where you were supposed to be. But more importantly, if I read anything there I know is wrong, or just badly written, whatever I'm supposed to be doing, I can barely keep myself from digging out the books and fixing it, just because I can. If Randall Munroe's "somebody is wrong on the internet" cartoon was inspired by anything it was Wikipedia.