Summernote is a really beautiful text editor on Bootstrap. It's simple, mobile friendly, and it comes with two different pre-styled themes (dark or light). It's easy to install and customize and it's totally open source.
Matheditor.js is a unique rich text editor that works like all of the other WYSIWYG editors on this list, only instead of using it to generate formatted text, you use it to generate formatted math equations. It uses the MathQuill library, is super lightweight, and is optimized for mobile use.
Froala is a lightweight WYSIWYG editor that is incredibly easy to use, supports dozens of different languages, is mobile friendly, and supports common keyboard shortcuts.
Redactor is a simple rich text editor with a clean, minimalist design. One of its coolest feature is a drag and drop capability for images.
Cazary is another simple WYSIWYG editor that can be used in a few different modes. Full mode includes tons of customization and formatting options for the text within the editor, while minimal mode only offers a few common ways to format text, such as the option to make the text bold, italic, or underlined.
ContentTools is a really cool editor that allows for inline editing, which means you can set certain parts of your pages to be editable areas, and users can edit them on the spot. It's easy to integrate with any site.
This rich text editor has a clean and simple design, but what makes it unique is that it provides users with an instant preview of the text they're creating and formatting. It's also super lightweight and easy to use.
TinyMCE is a plugin that provides you with a feature rich WYSIWYG editor that includes features such as file management, spell check, media embedding, and image editing. This powerful editor would be a great addition to any site.