MediaCMS is a modern, fully featured open source video and media CMS. It is developed to meet the needs of modern web platforms for viewing and sharing media. It can be used to build a small to medium video and media portal within minutes.

It is built mostly using the modern stack Django + React and includes a REST API.

Designed for personal use only. If you want people to be able to create accounts, you can do that. If you want it to be "everybody but me can only watch thigns," you can do that. If you want the entire install to be private and requiring logins from everybody, you can do that, too.

Postlight's Awesome CMS is a collection of open and closed source Content Management Systems (CMS) for your perusal.

HybridAuth is a library for web apps which implements social sign-ins (meaning, you authenticate with an existing social network account (like Twitter or Facebook)) rather than applying for an account with an e-mail address. Lets the user import established profiles and contact lists from the socnet account they logged in with. Also has a library of plugins for existing frameworks and applications, from Drupal to Concrete to Wordpress. Written in PHP so that it'll run on just about every webhost out there out of the box.

MDwiki is a single-page wiki engine implemented as an HTML5 page with JavaScript. Drop a bunch of Markdown documents into a directory, add a copy of mdwiki.html, and you now have a fully functional wiki. You can't edit it with your web browser but it's fine as a mostly static CMS or a documentation site.