Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
draw.io - Online diagramming application
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
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.
Invision - Prototyping and collaboration for design teams
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.