- The granddaddy of all Q&A sites (I think). Started in 2003 as an adjunct to MetaFilter, the community blog run by Matt Haughey. In the mid-2000s MetaFilter instituted a one-time $5 fee to join, in order to keep the community small and post quality high. It's worked, for the most part, but the MeFi community has some proclivities that sometimes leads to groupthink in the answers you see on AskMetaFilter. (That's not a knock on the site - I've been a member for years.)
- Famous around the Internet for it's massive userbase and generally poor answers. Certainly the best place to go if your primary interest is answer volume (as opposed to answer quality). Unlike MetaFilter, YA has a karma system of sorts that rewards high-quality answers. This doesn't actually seem to discourage bad answers, mind.
- Fluther's questions are broken down into browsable categories, but the goal of the site is to 'learn' what topics you're knowledgeable about and feed you questions that way. Launched a couple of years ago but judging from the number of responses the average question gets it hasn't picked up steam.
- Reportedly very popular with teens. Formspring questions are made from one user directly to another and don't create static URLs, so the focus is on answering questions within existing social networks. My perception could be off though, I haven't used Formspring that much myself.
- A relatively recent collection of themed Q&A sites built on the success of the programmer-oriented Stack Overflow. Questions are user-moderated with a reddit-like upvote/downvote system.
- I can't tell you anything from personal experience about Facebook Questions because I don't have access to it yet - it seems to still be rolling out. Link above is to Josh Lowensohn's take on CNet.
- Hunch: A people-powered recommendation engine.
- Aardvark: A people-powered semantic search engine.
- Ask Reddit allows joke answers, but if you don’t want them, add the “[serious]” tag to your question. Then, jokes will be weeded out by moderators. On the right-hand side of the ask-reddit home page, there are links to other reddit Q&A pages, such as one for relationship-advice and one for tech support. You also might enjoy reddit’s . This is where you can post a strong opinion (e.g. “Abortion is murder”) and ask people to try to change your mind about it.” It’s for people who really are open to having their mind’s changed—not for people who just want to argue or spread propaganda. And there’s , where you can ask people to explain complex ideas in simple words.