Cacoo - Create diagrams online Real time collaboration
Diagramly - Draw Diagrams Online
Frame Box - Lightweight online tool for creating mockups
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iPlotz: wireframing, mockups and prototyping for websites and applications
MockFlow: Online Wireframe Tool
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Wireframe and Mockups Software - Build Mockup , Wireframes and quick prototypes
Wireframe Software - Free website wireframe software by Gliffy
Wirify – The web as wireframes