Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
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.
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
Axure RP - The prototyping and specification software used by 87% of the Fortune 100. Beyond mockups, design interactive prototypes for business solutions, websites, and applications.
Wireframe.cc - Sweet and simple online wireframing tool.