Recommended and mentioned products
prismic.io is a web software you can use to manage content in any kind of website or app. API-driven.
Prismic - free up to a million api calls / 100gb in CDN usage.
Headless CMS and Application Framework built with Node.js, React and MongoDB
Payloadcms - free for a single user - no api limits - self hosted (optional) - and pretty damn clean and customizable. With a lot of great add ons.
The GraphQL Headless CMS
Graphcms - great api (graphql - gatsby fans rejoice!), free up to a million api calls / 100gb in assets.
A simple CMS for Jekyll and Hugo sites.
Forestry - - pretty much free, handles all types of data, decent api, and has a semi-simplistic ui.
Free and Open-Source Headless CMS
Directus - free open source option, clean ui, simple api, very client friendly.
Agility CMS is a cloud-based content management system for building and managing responsive websites and apps.
Agilitycms - free plan for developers - otherwise obscenely expensive - which sucks because their UI is the closest to wordpress that I’ve come across — so I know clients would use this as the bar and so sadly I never offer it to the smb clients.