Stackoverflow.com - Community driven Q&A site
According to them this is "programming Q&A site" for FREE but it is community driven. I have been member from the day it started but was never an active participant but recently decided to give it a try and I am addicted!! Yes, this is not yet-another-group but something entirely different. Interface of the site is excellent and notion of badge and rewards is efficient at keeping the trolls and noise at very low level.
You need to earn the right to be able to vote, approve or comment. It does not restrict you from putting questions or replying but depending on your answer's quality and timeliness poster can give you ranking and thus give you points. This simple concept forces people to answer precisely and correctly very fast before someone beats you to it!! Hence addiction :)
Give it a try!
Have a great weekend