Sporcle

If you're a fan of common knowledge, interesting facts, or logic puzzles, be warned; this post may lead to a new addiction.
So I've been playing Quizduel (it's this game on your smart phone where you choose a category and get 3 multiple choice questions in that category. Then you switch, and the other person first has to answer the same questions in your chosen category, and then gets to pick one of their own) with one of my colleagues for about a year now, and it's been fun, but at times also rather slow, as you have to wait for the other person to respond.
And then another colleague mentioned Sporcle. I'd totally forgotten about Sporcle, which is weird because I'd played some games on it a while ago, and it was great fun. But anyway, he mentioned that he had an account and that he spent some time on it whenever he felt like doing random Internet stuff (for me, mostly; while I'm on a train). And I went back, and remembered how great it was, and also made an account, and now I'm addicted.
I mean, what's not to like? There are literally millions of quizzes, divided into categories ranging from 'Just for fun' and 'Entertainment' to 'Literature' and 'Science'. But that's not all. They have different types of quizzes too. No standard multiple choice answers, but intricate logic puzzles where clues lead you through filling in all the squares, or sorting challenges where you have to think about what connects the elements you can choose from. Lots of picture quizzes with images or maps you can click on, but also quick quizzes which take just 1 minute to solve (or so they say... I never finish them in time).
And the best thing about having an account; you can see how well your friends did in a quiz, and challenge other players. So when I miraculously get 100% at my first attempt in some oddball quiz, I will challenge my colleague to see how well he does (depending on the subject, he's usually better or quicker). Also, you get to collect badges. Looots of badges. I now have 14, ranging from 'Newbie' (for signing up) via 'Sorting Hat' (for doing 25 Harry Potter quizzes) through to several badges earned for playing 15+ quizzes between specific times. There are 401 badges in total, some of which you only get for playing 1,000 quizzes of a certain category or type.
Now I am a slightly competitive person by nature (as long as it doesn't involve too much exercise), but even without the challenges and badges, just from the general quality and depth of the quizzes, Sporcle is really a thing to check out. Fun and frustration guaranteed. And if you feel the need for a challenge... my username is Dromera.