I’ve only visited a few of these sites. Straight Dope didn’t make the list. Maybe next year.
It’ll take me awhile to check all these sites out. I’ll skip over any pay sites. I don’t need to improve my life that much. 
Having answered a number of questions on Quora it is probably only one which I might have an issue with recommending anymore as its standards have really fallen in the last year or so. The others are all pretty good.
I don’t think that’s a great article. Couchsurfing makes you cleverer? Huh? I mean, you could use the same logic to say Expedia makes you cleverer (traveling and experiencing other cultures widends your horizons). Also, a few of those sites lock anything much useful behind paywalls and subscription models, e.g. Nerd Fitness and Lumosity.
I’d rather the “more clever than” (I was earlier) than “cleverer,” but your cleverness* may vary.
- doesn’t sound right either; like a rock outcropping in Scotland
One site I’d recommend is the YouTube channel Crash Course.
justin guitar (from the op link) has really good free lessons. It’s an honor system where you buy the leson dvds if you like the online lessons.
I’ve heard good things about code academy. I’m already a computer analyst/programmer and don’t need it.