draw.io - Online diagramming application
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
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.
Balsamiq Cloud - Fast, approachable, collaborative wireframing
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Justinmind - Define awesome experiences through wireframes & prototypes.