Quora
Quora has a Upvote/Downvote system. If you like an answer, you can upvote it and if you do not like it you can downvote it. Any user can upvote or downvote a question' answer no matter whether he asked that question or not. Quora has some strict policies to fight with spam and people posting copied content. If you do not follow them, your account will be suspended by the Quora Moderation team.