Best Twitter Communities to Join

Twitter is an American microblogging and social networking service on which users post and interact with messages known as "tweets". Registered users can post, like and retweet tweets, but unregistered users can only read them. Twitter has a new community feature where user's tweets in a group are aggregated in one place, and are read-only to people outside of the community. Prior to the launch of this community feature, communities on Twitter would organize under common hashtags used in their tweets.