Edtr.io - Intuitive WYSIWYG web editor, open source & fully adaptable.
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.
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
draw.io - Online diagramming application
MathType - The Professional version of Equation Editor- MathType is a powerful interactive tool for Windows and Macintosh that lets you create mathematical notation for word processing, web pages, desktop publishing, presentations, and for TeX, LaTeX, and Math…
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.