There seem to be new ones popping up all the time, so a full list would be never ending... The ones I use the most are Blurtit and Quora to be honest. Occasionally I land on Yahoo Answers when searching for information online too.
I also use the Q&A app Jelly which is kind of fun, and I'm eager to try out Cinch - although that's available in the US only at the moment (poor little UK is always left behind).
Working for Blurtit, I obviously spend most of my day on here... And I know we have some way to go to catch up with the big players. Having said that, we've got some exciting plans ahead, and it's worth noting that Blurtit first launched in 2006 and had a very active community at one point - so in many ways we are the originals :)