draw.io - Online diagramming application
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
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 V2 - A beautiful modern WYSIWYG HTML text editor
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Edtr.io - Intuitive WYSIWYG web editor, open source & fully adaptable.