I want to ask about various mathematical fora and discussion boards available online. I think it might be useful to have such a list here at Math.SE.
If I may suggest, it could be useful to keep one long list with basic information as a community wiki, and if you want add some additional information on some of them, a separate answer might be the best way. (E.g. information like: "this forum is particularly suitable for questions about contest math" or "this forum is frequented by specialists in computer algebra, most discussions are about CAS software such as GAP", etc.)
In particular, if you deem some of other fora more suitable for some kind of questions than math.SE, this would probably be an interesting information for users of this site.
A related question: Which discussion board is good for homework questions?