I'd be surprised if there are any of us who don't have some gap in knowledge that a majority of the rest of us found surprising. But really I can't think of any knowledge of this type I'm missing that I can't just look up (rather than ask here) if I realize that I don't have it. (Things of this type I can recall looking up in the past few years: ordering at a bar, dialing international phone numbers, reaching someone at a phone extension, getting a cashier's check from a bank, how to properly wear a suit jacket, how to read facial expressions and make small talk.)

I like wikihow, ehow, and similar sites--and I also find that guides intended for recent immigrants or people with autism are useful for "things everyone is supposed to know".