A recent discussion reminded me of a crowd-sourced trivia game that I built a few years ago. It's very simple: anyone can add questions labeled with a category, and when you play, each "card" has 5 questions on it. You can play it on your own, but it's more fun with a group. Just like with Trivia Pursuit, one person or team is answering and another is reading the card. You can let the answerer pick a category, and then read them the question.
I put a password access on it, to keep the spam bots out. Like it says in the message: username is "play", password is "now". Try it out:
There's some elegance in the implementation. The questions you've written are saved in your session, so you can get a list of them and edit them. The questions you've answered are there too, so you won't get the same question twice. Let me know if you have a use for it or want more features!