Sigil - Sigil is a multi-platform WYSIWYG ebook editor. It is designed to edit books in ePub format.
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.
Jutoh - Epub editor, convertor and creator software for Mac and PC.
draw.io - Online diagramming application
calibre - Ebook manager, viewer & converter
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.