Free & Open Source Contentful Alternatives
Contentful is an API driven content platform. This sounds like a bunch of buzzwords, but essentially it means that you will create your content items in one interface. That information is then openly available to content publishing tools that can be web sites, web applications or mobile applications. It's often also used in decoupled CMS scenarios.
Contentful is a Software as a Service (SaaS) platform where you pay for the right to use the software online. It is very convenient to use as there are absolutely no technical tasks on top of the monthly fee. This opens up your resources to maintaining and developing the vessel that is used to display all of that content.
CaaS (Content as a Service) platforms like Medium certainly have their appeal, but they can also be limiting. Certain features that you may expect after using an Open Source CMS might not be there and you've got no control over that development. In addition there are arbitrary limitations in the number of content items in the content repository and so on.
In these cases you might want to look into using an Open Source CMS with a REST API. This is sometimes known as headless CMS and so on. It adds overhead to your technical development team, but also opens up extension possibilities that are simply not there for fully managed systems.
Some options for running your own CaaS are Drupal, eZ Platform and WordPress. All of these have their strengths and weaknesses, so before you select one make sure the content permissions system, multilingual capabilities are up there with your needs. Another aspect that is highlighted in CaaS use is the support for "Chunky Content" in a CMS.
One advantage of the hosted model is obviously the care free promise of SaaS, but running your own CaaS using Docker makes this less of an issue as cloud deployments are democratized.
Update: In 2018 the landscape has changed, with the best free alternative to Contentful is the GraphQL powered GraphCMS, a scalable option to hosted CaaS.