I knew nothing about this site before the keynote for the MIX09 conference, but decided to try it out. Stackoverflow.com is a community of developers centered around answering each other's questions. Like many other forums you can ask a question and people will give you answers. Unlike many other forums users can rate answers and questions, and these ratings affect the reputation of the person that provided the answer/question. So when you get an answer you have a hint of the person's credibility. Rather than organizing questions in forums questions have tags. This solves a limitation of hierarchial organizationsof questions (sometimes a question would fit in to more than one hierarchial classification but you can only place it in one). When browsing questions you can choose the tags that interest you and the view is filtered.