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?
English language
Of course, there is mathoverflow, many users of this site are active there too. google: mathoverflow site:math.stackexchange.com
sci.math, Usenet group, google: "sci.math" site:stackexchange.com
AoPS - Art of Problem Solving, phpBB, supports TeX using LatexRender, google: artofproblemsolving site:stackexchange.com
MHF - mathhelpforum, supports TeX, google: mathhelpforum site:stackexchange.com
mymathforum, phpBB, suports TeX, google: mymathforum site:stackexchange.com
Physics Forums - Sub-forum Mathematics, google: physicsforums site:math.stackexchange.com
S.O.S. Mathematics CyberBoard, phpBB, supports TeX using LatexRender, google: sosmath site:stackexchange.com
Topology Q+A Board, google: "yorku.ca" site:stackexchange.com
Free Math Help, google: "freemathhelp.com" site:stackexchange.com
Math Help Boards, google: "mathhelpboards.com" site:stackexchange.com
Tetration forum(about) and Forum itself (moderated, all levels,phpBB,TeX), google: tetrationforum site:stackexchange.com
Mathematics subforum of xkcd; phpBB; supports LaTeX using jsMath - but they prefer avoiding it where possible, so that the site is not slowed down, see here; google: "fora.xkcd.com" site:stackexchange.comThis forum no longer exists.Abstract and Linear Algebra. , Google: "math.miami.edu/forum" site:stackexchange.com
Purplemath, phpBB, supports LaTeX; Google: "purplemath.com" site:stackexchange.com
Wikipedia Reference desk Mathematics
Integrals and Series, users communicate in TeX while the interface does NOT render.
German language
de.sci.mathematik, Usenet group
Matroids Matheplanet, supports LaTeX
Czech language
Italian language
Russian language