Useful mathematical fora 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?
 A: English language

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*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.com This 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

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*de.sci.mathematik, Usenet group


*Matroids Matheplanet, supports LaTeX
Czech language

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*Matematické Fórum at matematika.cz (formerly matweb.cz); supports TeX

Italian language

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*Matematicamente.it/forum, phpBB, supports LaTeX using MathJax; Google: matematicamente.it/forum site:stackexchange.com.
The forum also has an English language section (called English Corner).

Russian language

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*dxdy, phpBB, supports LaTeX.

