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

A bare-bones server that implements a universal back-end for a content management system. Works a little like a key/value store, or a straight JSON store. Implements a JSON HTTP API. Set up the database on the command line by giving it the data spec and then write your code to refer to it. Supports HTTP/2 and Let's Encrypt natively. Can also generate basic HTML for implementing the front-end.

Octopress is a framework for generating static sites that can then be uploaded to the Github Pages service to provide websites for hosted projects. Octopress provides HTML, Javascript and CSS templates for the project site, all you have to do is add your own content and check it in.

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.