MathJax is available through a Content Distribution Network (CDN) so that
anyone can include it in their web pages without having to download or
install it themselves.
The CDN receives over 100 million unique visitors a month.
We have a
third-party contributed extensions
site where users can submit their own extensions for others to use. The
extensions are available on the CDN.
The code for MathJax is available on GitHub. Anyone can fork it, modify
it, and contribute back (we have a CLA for contributions to the
core). See the Network Diagram.
The localization of MathJax is facilitated by TranslateWiki.