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.
Documentator - We make documenting your product a piece of cake 📘
draw.io - Online diagramming application
UnMarkDocs - Unleash the Power of Markdown to Write Better Product Documentation
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
You Need A Wiki - Create a wiki with Google Docs