When I started roaming the internet back in 2003 most communities I visited were using discussion boards. For discussions they were perfectly fine and are, in my opinion, still unrivaled. But they were of course also used for sharing trivialities and problem solving. While the former works well enough, the latter does not work well on boards at all. Assume you post a question and receive a number of disagreeing answers from experts that then argue in a language that you can not understand at all. How do you find out which answer is the best? How do you keep track of the pages-long discussion?
StackExchange provides a very good alternative for this. Here, both questions and answers can be down- and upvoted, thus creating a measure of confidence the community has in a particular statement. You can change your vote later if a discussion in the comments changes your mind or answers are edited. Futhermore, answers are sorted by score, not date. Therefore, you can visually observe convergence towards a meaningful answer assuming that at least some people around know what they are talking about.
A reputation based reward system keeps people behaving; you earn reputation by getting others to upvote what you write and lose it by getting downvoted or downvoting yourself. You can also set a bounty on questions whose answers you are interested in and hope that somebody is motivated enough by some reputation points to take the time to answer. The influence of trolls and spammers is kept minimal since many features (e.g. closing, retagging) are only available if you have certain amount of reputation; the more impact the feature has the more reputation you need. This is necessary because there are no moderators or the like; each community manages its site on its own.
The most notable subsite is Stack Overflow which has been around quite a while, but a number of other sections has come up lately. They cover a broad range of topics from Cooking and Bicycles over TeX – LaTeX and Ubuntu to Mathematics and Theoretical Computer Science. Check it out; even if you do not ask questions yourself, reading and answering those that match you interests can be both interesting and entertaining.