draw.io - Online diagramming application
Typora - A minimal Markdown reading & writing app.
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.
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.