Invision - Prototyping and collaboration for design teams
draw.io - Online diagramming application
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
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.
Zeplin - Collaboration app for UI designers & frontend developers
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.