CKEditor - Real-time collaborative future-ready rich text editor
WordPress - WordPress is web software you can use to create a beautiful website or blog. We like to say that WordPress is both free and priceless at the same time.
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
ApostropheCMS - Apostrophe is a powerful website builder platform built on an enterprise open source CMS, offering in-context live editing, visual design tools, and multisite enablement in a full stack JS environment.
Trumbowyg - Feature-packed WYSIWYG editor in only 15kB of JavaScript
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.