UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
draw.io - Online diagramming application
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
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.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.