CKEditor - Real-time collaborative future-ready rich text editor
draw.io - Online diagramming application
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Froala Editor - Froala Editor is a WYSIWYG HTML editor that enables rich text editing capabilities for the applications.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.