Apache is a free and open-source cross-platform web server software.
cdnjs is a free distributed JS library delivery service.
Google Tag Manager is a tag management system (TMS) that allows you to quickly and easily update measurement codes and related code fragments collectively known as tags on your website or mobile app.
MathJax is a cross-browser JavaScript library that displays mathematical notation in web browsers, using MathML, LaTeX and ASCIIMathML markup.
Varnish is a reverse caching proxy.
YouTube is a video sharing service where users can create their own profile, upload videos, watch, like and comment on other videos.