I've known a number of stupid people, but it's almost entirely through indirect contact (the fact that I'm rather introverted and don't meet new people often likely has some effect on that, mind). Through the internet, we discover the wonderful effects anonymity has on people. Individuals who might otherwise be perfectly polite and willing to be reasonable are no longer constrained to do so by subtle societal pressures. Some take this too far and believe that it absolves them of the idea of polite interaction. Now, manners don't make a person smart. The problem is, people often make the reverse mistake: if you're rude and "refuse to sugar coat" something, you're clearly smart. You are the rational, sharp minded cynic amongst a world of sheep and you refuse to protect their fragile little egos against the blunt force of you incredible brain and desire for truth! This isn't a sensible position, but it's one that shows up more than I care to see.
So there's that.
The other bits of stupidity I see tend to involve politics. I was never very involved in politics. I started getting involved around the SOPA/PIPA debates and my frustration for incredibly dumb political maneuvers, many only performed out of the need to be seen doing something no matter how counterproductive, has exposed me to a whole new world of logical fallacies, stubborn refusal to acknowledge stupidity and a willingness to contort truth into shapes that should never have been seen without heaping doses of sarcasm and bitterness.
That said, I would like to point out the importance of differentiating stupidity and ignorance. I normally don't hold ignorance against someone. You can't always be expected to know something, and it's idiotic to assume people can somehow just pull new information out of thin air without context. Actual stupidity is not a lack of information. Sadly, I see alot of this as well.
