Recs.
Updated
Stack Exchange sites (most famously Stack Overflow and Super User) allow users to ask and answer questions around a variety of topics. Individual sites are dedicated to specific topics, with meta-tags allowing refined categorisation within that subject area.
Specs
Pros
Pro Subject Matter: Various (site specific)
Whilst sites under the Stack Exchange banner are focussed on specific topics, there are a wide variety of such sites.
As such Stack Exchange could be described as being open to questions on any subject matter, but has the advantage that within its sub-sites, questions are targeted to the specific users/audiences of those sites.
Pro Experts can edit posts directly
High level users in a particular topic can improve quality of the content by editing. The edit can also be rolled back by the one that originally posted the edited post. Generally this improves the quality of the entire questions/answers pool and makes this an excellent source of information. It's like Wikipedia driven by questions.
Pro User ratings by topic
Users are scored based on their contributions, with the context of those contributions also being scored (i.e. answering a question tagged as being about XML increases that user's XML score). This results in allowing the questioner to gauge a rough idea of the answerer's relative knowledge within a domain when considering their answers vs contradictory answers from other users.