Drupal
Who in the world is using Drupal built sites, let's see....
Drupal was started by Dries Buytaert . You can learn more about Dries on Wikipedia if you like.
I started using Drupal back in 2008. Everyone said it was hard, but it was easy for me. Guess some sees things one way and I see them differently. So, what is hard to others to build a site with, it was easy for me.
10 years went by, I still built sites out of Drupal for the clients who want to blog regularly. Otherwise, if the client has no blogging need and doesn't have time to update the site, it is best to build the site in HTML. Less things to do for the client. However, we have clients who want to upkeep the site on their own and don't mind, then Drupal is recommended. Some might want to use WordPress and that is another story.