I've recently discovered Yahoo Answers.
Yes, I know it's been around forever, and as a self-respecting computer geek I really should have known about it before. I actually did know about it, but had never taken a look at it. I finally took a look at it, and saw a question I knew how to answer. So I answered it.
Now I'm hooked. I think it's worse than an online game, because when you get hooked on an online game you get to feel bad for being hooked on a game. Eventually the self-loathing gets to the point where you can break away from the game and do something else. But with Yahoo Answers, you're actually helping people.
It's hard to feel bad about helping people. Even if you're doing it online. A lot.