Well, I think it'd be useful to collect right here all the websites that we consider useful (this means, with LaTeX; a math forum without LaTeX is not a math one).
MathLinks
FMAT (spanish forum, I take part there )
S.O.S. Math
Post useful sites that you know!
You know, a math site that has been around for quite a while and has helped a lot is mathnerds.com
The thing is, considering the traffic they get, they absolutely refuse to implement LaTex. Why, I don't know?.
I asked why and one reply I got was it "cost too much". Come on
You still have to type it out the primitive, behind-the-times way, just using what's on the keyboard.
That site works differently than most. You have to be approved to answer questions which are posted by first answering some test questions in the category you wish to answer questions in.
Other sites with Latex:
Foro de Geometría
http://www.maths-forum.com/index.php? (in French)
Math Is Fun Forum
Physics Help and Math Help - Physics Forums (contain also math topics)
Problema sÄƒptÄƒmÃ¢nii (in Romanian)