I've noticed that very recently there has been a new site, mathematica.stackexchange.com.
It appears to be fairly new and very similar to this site.
What's the difference? It doesn't appear too obvious to me.
The Mathematica.SE website is dedicated for that software and its uses. This website is not for questions about mathematical software, as much as it is about mathematics.
(It's also not very new, with the first question from 4 years ago.)