ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
draw.io - Online diagramming application
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
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.
Wireframe.cc - Sweet and simple online wireframing tool.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.