Drupal is a free and open-source web content management framework.
Nginx is a web server that can also be used as a reverse proxy, load balancer, mail proxy and HTTP cache.
Pantheon is a WebOps (Website Operations) and Management Platform for WordPress and Drupal.
ShareThis provides free engagement and growth tools (e.g., share buttons, follow buttons, and reaction buttons) for site owners.
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.