Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
CKEditor - Real-time collaborative future-ready rich text editor
Xamarin.Android - Integrated environment for building not only native Android but iOS and Windows apps too.
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
Kodular - Much more than a modern app creator without coding
Froala Editor - Froala Editor is a WYSIWYG HTML editor that enables rich text editing capabilities for the applications.