Invision - Prototyping and collaboration for design teams
draw.io - Online diagramming application
Atomic - The fastest way to design beautiful interactions
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.
Pixelapse - Visual version control and collaboration workflow for open source and private design projects.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.