Not big on games or puzzles and the like, so mine's a weird and, thankfully, obscure webgaff called Whitechapel.
No, really. This is the only forum, BB, social media temporal-sinkhole that I: a) visit regularly (OK, daily hourly), and b) have ever posted to. (I know, even on the internet I still need to get out more.)
So the content occasionally means you'll always need to keep some brain bleach handy, but its inhabitants are way better-informed, educated, creative and enlightened than your average netizen. Not to mention, even without the threat of arse eels and urethral maggots, the overwhelming majority don't need to be reminded, gently coerced or bitch-slapped into behaving like a fucking civilised human being.
I'm fairly addicted to the SCP Foundation. It's a wiki about an organization that collects weird stories (written as if they were real) about objects, creatures, anomolies, etc. It's fun picking up on the story that runs through some of them.
www.aq.com cute MMORPG game. (flash based) It has ALL THE GEEK/NERD REFERENCES. All of them. Support and Dev take great care to make the games all ages safe. Easy to play with your kid(s)